Spinal mobility preservation method

ABSTRACT

A spinal mobility preservation apparatus and methods are disclosed. The spinal mobility preservation apparatus may include a proximal body, an intermediate body, a distal body, and an expandable membrane. The proximal body and the distal body secure the mobility preservation apparatus to adjacent vertebral bodies. At least one of an intermediate body and an expandable membrane secure the proximal body to the distal body and provide a degree of support to a spinal motion segment defined by the adjacent vertebral bodies. A single proximal body and an expandable membrane may also compose a spinal mobility preservation apparatus. The proximal body secured to one of a superior or an inferior vertebral body and the expandable membrane extending into the intervertebral disc space to support the spinal motion segment.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present Utility patent application is a continuation of U.S. patentapplication Ser. No. 10/972,040 filed Oct. 22, 2004 now U.S. Pat. No.7,662,173, which claims priority and benefits from and commonly assignedU.S. Provisional Patent Application Nos. 60/558,069, filed Mar. 31, 2004and 60/513,899, filed Oct. 23, 2003, the disclosures of which are herebyincorporated by reference, and which is a continuation-in-part of U.S.patent application Ser. No. 10/309,416 filed on Dec. 3, 2002, now U.S.Pat. No. 6,921,403, which is a continuation-in-part of U.S. patentapplication Ser. No. 10/125,771 filed on Apr. 18, 2002, now U.S. Pat.No. 6,899,716, which is a continuation-in-part of U.S. patentapplication Ser. No. 09/848,556 filed on May 3, 2001, now U.S. Pat. No.7,014,633, which is a continuation-in-part of U.S. patent applicationSer. No. 09/782,583 filed on Feb. 13, 2001, now U.S. Pat. No. 6,558,390,which, in turn, claims priority and benefits from U.S. ProvisionalPatent Application Ser. No. 60/182,748 filed on Feb. 16, 2000, each ofwhich are incorporated in their entirety into this disclosure byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to surgically implantable devices and,more particularly, to an apparatus and method for supporting the spine.

2. Description of the Related Art

Chronic lower back pain is a primary cause of lost work days in theUnited States. It is also a significant factor affecting both workforceproductivity and health care expense. Therapeutic procedures foralleviating back pain range from conservative methods, e.g., withintermittent heat, rest, rehabilitative exercises, and medications torelieve pain, muscle spasm, and inflammation, to progressively moreactive and invasive surgical methods which may be indicated if thesetreatments are unsuccessful, including various spinal arthroplasties,and eventually even spinal arthrodesis, i.e., surgical fusion.

There are currently over 700,000 surgical procedures performed annuallyto treat lower back pain in the U.S. In 2004, it is conservativelyestimated that there will be more than 200,000 lumbar fusions performedin the U.S., and more than 300,000 worldwide. These procedures representapproximately a $1 billion endeavor in an attempt to alleviate patients'pain. In addition, statistics show that only about 70% of theseprocedures performed will be successful in achieving this end.

Approximately 60% of spinal surgery takes place in the lumbar spine, andof that portion approximately 80% involves the lower lumbar vertebraedesignated as the fourth lumbar vertebra (“L4”), the fifth lumbarvertebra (“L5”), and the first sacral vertebra (“S1”). Persistent lowback pain is often attributable to degeneration of the disc between L5and S1. Traditional, conservative methods of treatment include bed rest,pain and muscle relaxant medication, physical therapy or steroidinjection. Upon failure of conservative therapy, spinal pain hastraditionally been treated by surgical interventions. These surgerieshave included spinal arthroplasty; arthrodesis, or fusion, which causethe vertebrae above and below the disc to grow solidly together and forma single, solid piece of bone. Yet, statistics show that only about 70%of these procedures performed will be successful in relieving pain.

There are multiple causes for a patient's lower back pain. The pain isfrequently hypothesized to arise one or more of the following: bulgingof the posterior annulus or PLL with subsequent nerve impingement;tears, fissures or cracks in the outer, innervated layers of theannulus; motion induced leakage of nuclear material through the annulusand subsequent irritation of surrounding tissue in response to theforeign body reaction, or facet pain. Generally, it is believed that 75%of cases are associated with degenerative disc disease. In cases ofdegenerative disc disease, the intervertebral disc of the spine suffersreduced mechanical functionality typically due to dehydration of thenucleus pulposus. Surgical procedures, such as spinal fusion anddiscectomy, may alleviate pain, but do not restore normal physiologicaldisc function attributable to healthy anatomical form, i.e., intact discstructures such as the nucleus pulposus and annulus fibrosis, asdescribed below.

The spinal column or backbone encloses the spinal cord and consists of33 vertebrae superimposed upon one another in a series which provides aflexible supporting column for the trunk and head. The vertebraecephalad (i.e., toward the head or superior) to the sacral vertebrae areseparated by fibrocartilaginous intervertebral discs and are united byarticular capsules and by ligaments. The uppermost seven vertebrae arereferred to as the cervical vertebrae, and the next lower twelvevertebrae are referred to as the thoracic, or dorsal, vertebrae. Thenext lower succeeding five vertebrae below the thoracic vertebrae arereferred to as the lumbar vertebrae and are designated L1-L5 indescending order. The next lower succeeding five vertebrae below thelumbar vertebrae are referred to as the sacral vertebrae and arenumbered S1-S5 in descending order. The final four vertebrae below thesacral vertebrae are referred to as the coccygeal vertebrae. In adults,the five sacral vertebrae fuse to form a single bone referred to as thesacrum, and the four rudimentary coccyx vertebrae fuse to form anotherbone called the coccyx or commonly the “tail bone”. The number ofvertebrae is sometimes increased by an additional vertebra in oneregion, and sometimes one may be absent in another region

Each vertebra has a spinous process, which is a bony prominence behindthe spinal cord that shields the spinal cord's nerve tissue. Thevertebrae also have a strong bony “vertebral body” in front of thespinal cord to provide a platform suitable for weight-bearing. Eachvertebral body has relatively strong, cortical bone layer comprising theexposed outside surface of the body, including the endplates, andweaker, cancellous bone comprising the center of the vertebral body. Thebodies of successive lumbar, thoracic and cervical vertebrae articulatewith one another and are separated by the intervertebral discs. Eachintervertebral disc comprises a fibrous cartilage shell enclosing acentral mass, the “nucleus pulposus” (or “nucleus” herein) that providesfor cushioning and dampening of compressive forces to the spinal column.The shell enclosing the nucleus comprises cartilaginous endplatesadhered to the opposed cortical bone endplates of the cephalad andcaudal vertebral bodies and the “annulus fibrosis” (or “annulus” herein)comprising multiple layers of opposing collagen fibers runningcircumferentially around the nucleus pulposus and connecting thecartilaginous endplates. The natural, physiological nucleus is comprisedof hydrophilic (water attracting) mucopolysaccharides and fibrousstrands of protein polymers. In a healthy adult spine, the nucleus isabout 80% water by mass. The disc is a hydrostatic system. The nucleusacts as a confined fluid within the annulus. It converts compressive onthe vertebral end plates (axial loads) into tension on the annulusfibers. The nucleus is relatively inelastic, but the annulus can bulgeoutward slightly to accommodate loads axially applied to the spinalmotion segment.

The intervertebral discs are anterior to the spinal canal and locatedbetween the opposed end faces or endplates of a cephalad and a caudalvertebral bodies. The inferior articular processes articulate with thesuperior articular processes of the next succeeding vertebra in thecaudal (i.e., toward the feet or inferior) direction. Several ligamentshold the vertebrae in position yet permit a limited degree of movement.The ligaments include the supraspinous, the interspinous, the anteriorand the posterior longitudinal, and the ligamenta flava. The assembly oftwo vertebral bodies, the interposed, intervertebral, spinal disc andthe attached ligaments, muscles and facet joints is referred to as a“spinal motion segment”. In essence, the spine is designed so thatvertebrae “stacked” together can provide a movable support structurewhile also protecting the spinal cord's nervous tissue that extends downthe spinal column from the brain.

The spine has defined range of motion. Its range of motion can bedescribed in terms of degrees of motion. More particularly, the spinesrange of motion is typically described relative to translation androtation about three orthogonal planes relative to an instantaneouscenter of rotation around the vertical axis of the spine. This cangenerally be broken down into six degrees of motion. These includeflexion, extension, compression, rotation, lateral bending, anddistraction.

Flexion and extension of the spine combine forward sliding and rotationof the vertebrae. The facet joints and the annulus resist the forwardsliding. Rotation is resisted by the annulus; capsules of the facetjoints; action of the back muscles, and passive tension generated by thethoracolumbar fascia. Extension is resisted by the facet joints, andsecondarily by the annulus. The spine is typically resistant to injuryif the force is only in pure flexion, as the combination of the facetjoints and disc are intrinsically stable in this plane. While the spinalmuscles can be injured during forceful flexion since they are importantin controlling this motion, ensuing pain is not typically chronic.Extension is impaired by impaction of the facet joints and eventuallythe inferior articular process against the lamina. This can result in acartilage injury of the facet joint; disruption of the facet capsule,and facet joint or pars interarticularis fracture.

Compression of the spine is due to body weight and loads applied to thespine. Body weight is a minor compressive load. The major compressiveload on the spine is produced by the back muscles. As a person bendsforward, the body weight plus an external load must be balanced by theforce generated by the back muscles. That is, muscle loads balancegravitational loads so that the spine is in equilibrium, to preclude usfrom falling over. The external force is calculated by multiplying theload times the perpendicular distance of the load from the spine. Inessence, the further the load is from the spine, the larger thecompressive load is on the spine. Since the back muscles act close tothe spine, they must exert large forces to balance the load. The forcegenerated by the back muscles results in compression of spinalstructures. Most of the compressive loads (˜80%) are sustained by theanterior column which includes the intervertebral discs and thevertebral bodies.

Compression injuries occur by two main mechanisms. It generally occursby either axial loading by gravity or by muscle action. Gravitationalinjuries result from a fall onto the buttocks while muscular injuriesresult from severe exertion during pulling or lifting. A seriousconsequence of the injury is a fracture of the vertebral end plate.Since the end plate is critical to disc nutrition, an injury can changethe biochemical and metabolic state of the disc. If the end plate heals,the disc may suffer no malice. However, if the end plate does not heal,the nucleus can undergo harmful changes. The nucleus loses itsproteoglycans and thus, its water-binding capacity. The hydrostaticproperties of the nucleus are compromised. Instead of sharing the loadbetween the nucleus and the annulus, more load is transferred to theannulus. The annulus fibers then fail. In addition to annular tears, thelayers of the annular separate (delaminate). The disc may collapse or itmay maintain its height with progressive annular tearing. If the annulusis significantly weakened, there may be a rupture of the disc wherebythe nuclear material migrates into the annulus or into the spinal canalcausing nerve root compression.

Rotation of the spine is accomplished by the contraction of theabdominal muscles acting through the thorax and the thoracolumbarfascia. There are no primary muscles responsible for lumbar rotation.The facet joints and the collagen fibers of the annulus resist thisrotation. In rotation, only 50% of the collagen fibers are in tension atany time, which renders the annulus susceptible to injury.

The spine is particularly susceptible to injury in a loading combinationof rotation and flexion. Flexion pre-stresses the annular fibers. As thespine rotates, compression occurs on the facet joint surfaces of thejoint opposite the rotation. Distraction occurs on the facet joint onthe same side of the rotation. The center of rotation of the motionsegment shifts from the back of the disc to the facet joint incompression. The disc shifts sideways and shear forces on the annularfibers are significant. Since the annular fibers are weak in thisdirection, they can tear. If the rotation continues, the facet jointscan sustain cartilage injury, fracture, and capsular tears while theannulus can tear in several different ways. Any of these injuries can bea source of pain.

Lateral bending is a combination of lateral flexion and rotation throughthe annulus and facet joints.

Pure distraction rarely occurs and is usually a combination of tensionand compression on the spinal joints depending on the direction ofapplied force. An example of a distraction force is therapeutic spinaltraction to “unload” the spine. In the context of the present invention,the term distraction may refer procedurally to an elevation in heightthat increases the intervertebral disc space 860 resulting during orfrom introduction of a spinal implant. Temporary distraction willgenerally refer to elevation of disc height by an apparatus which issubsequently removed but wherein the elevation is retainedintra-operatively, while the patient remains prone. Thus, an implant maybe inserted into an elevated disc space 860 first created by anotherapparatus, and thereafter physical presence and dimensionality of theimplanted apparatus would preserve the level of distraction.

Prior devices have typically not preserved, restored or otherwisemanaged these six ranges of motion. Accordingly, a need exists forapparatus and methods for preserving, restoring, and/or managingmobility of the spine.

As noted above, the nucleus pulposus that forms the center portion ofthe intervertebral disc consists of 80% water that is absorbed by theproteoglycans in a healthy adult spine. With aging, the nucleus becomesless fluid and more viscous and sometimes even dehydrates and contractscausing severe pain in many instances. This is sometimes referred to as“isolated disc resorption”. The intervertebral discs serve as“dampeners” between each vertebral body. They generally function tominimize the impact of movement on the spinal column. Intervertebraldisc degeneration, which may be marked by a decrease in water contentwithin the nucleus, can render the intervertebral discs ineffective intransferring loads to the annulus layers. In addition, the annulus tendsto thicken, desiccate, and become more rigid with age. This decreasesthe ability of the annulus to elastically deform under load which canmake it susceptible to fracturing or fissuring. One form of discdegeneration occurs when the annulus fissures or is torn. The fissuremay or may not be accompanied by extrusion of nucleus material into andbeyond the annulus. The fissure itself may be the sole morphologicalchange, above and beyond generalized degenerative changes in theconnective tissue of the disc, and disc fissures can nevertheless bepainful and debilitating. Biochemicals contained within the nucleus areenabled to escape through the fissure and irritate nearby structures.

A fissure also may be associated with a herniation or rupture of theannulus causing the nucleus to bulge outward or extrude out through thefissure and impinge upon the spinal column or nerves. This is commonlytermed a “ruptured” or “slipped” disc. Herniations may take a number offorms. With a contained disc herniation, the nucleus may work its waypartly through the annulus but is still contained within the annulus orbeneath the posterior longitudinal ligament, and there are no freenucleus fragments in the spinal canal. Nevertheless, even a containeddisc herniation is problematic because the outward protrusion can presson the spinal cord or on spinal nerves causing sciatica.

Another disc problem occurs when the disc bulges outwardcircumferentially in all directions and not just in one location. Thisoccurs when, over time, the disc weakens bulges outward and takes on a“roll” shape. Mechanical stiffness of the joint is reduced and thespinal motion segment may become unstable, shortening the spinal cordsegment. As the disc “roll” extends beyond the normal circumference, thedisc height may be compromised, and foramina with nerve roots arecompressed causing pain. Current treatment methods other than spinalfusion for symptomatic disc rolls and herniated discs include“laminectomy” which involves the surgical exposure of the annulus andsurgical excision of the symptomatic portion of the herniated discfollowed by a relatively lengthy recuperation period. In addition,osteophytes may form on the outer surface of the disc roll and furtherencroach on the spinal canal and foramina through which nerves pass. Thecephalad vertebra may eventually settle on top of the caudal vertebra.This condition is called “lumbar spondylosis”.

Various other surgical treatments that attempt to preserve theintervertebral spinal disc and to simply relieve pain include a“discectomy” or “disc decompression” to remove some or most of theinterior nucleus thereby decompressing and decreasing outward pressureon the annulus. In less invasive microsurgical procedures known as“microlumbar discectomy” and “automated percutaneous lumbar discectomy”,the nucleus is removed by suction through a needle laterally extendedthrough the annulus. Although these procedures are less invasive thanopen surgery, they nevertheless suffer the possibility of injury to thenerve root and dural sac, perineural scar formation, re-herniation ofthe site of the surgery, and instability due to excess bone removal. Inaddition, they generally involve the perforation of the annulus.

Although damaged discs and vertebral bodies can be identified withsophisticated diagnostic imaging, existing surgical interventions soextensive and clinical outcomes are not consistently satisfactory.Furthermore, patients undergoing such fusion surgery experiencetypically have significant complications and uncomfortable, prolongedconvalescence. Surgical complications may include disc space 860infection, nerve root injury, hematoma formation, instability ofadjacent vertebrae, and disruption of muscles, tendons, and ligaments.

Any level of the spine can be affected by disc degeneration. When discdegeneration affects the spine of the neck, it is referred to ascervical disc disease, while when the mid-back is affected, thecondition is referred to as thoracic disc disease. Disc degenerationthat affects the lumbar spine causes pain localized to the low back andis sometimes common in older persons and known as lumbago Degenerativearthritis (osteoarthritis) of the facet joints is also a cause oflocalized lumbar pain that can be diagnosed via x-ray analysis.

Radiculopathy refers to nerve irritation caused by damage to the discbetween the vertebrae. This occurs because of degeneration of theannulus fibrosis of the disc, or due to traumatic injury, or both.Weakening of the annulus may lead to disc bulging and herniation. Withbulging and herniation, the nucleus pulposus can rupture through theannulus and abut the spinal cord or its nerves as they exit the bonyspinal column. When disc herniation occurs, the rupture of the nucleuspulposus into or through the annulus may irritate adjacent nervoustissue, causing local pain, or discogenic pain, in the affected area.

The pain from degenerative disc or joint disease of the spine may betreated conservatively with intermittent heat, rest, rehabilitativeexercises, and medications to relieve pain, muscle spasm, andinflammation. However, if these treatments are unsuccessful,progressively more active interventions may be necessary. These moreactive interventions may include spinal arthroplasty includingprosthetic nucleus device implantation; annulus repair, and total discreplacement, and eventually, even spinal arthrodesis. The nature of theintervention performed depends on the overall status of the spine, andthe age and health of the patient. In some cases, the intervention mayinclude the removal of the herniated disc with laminotomy (a small holein the bone of the spine surrounding the spinal cord), a laminectomy(removal of the bony wall), percutaneous discectomy (removal by needletechnique through the skin), chemonucleolysis (various chemicaldisc-dissolving procedures), among other procedures.

When narrowing of the spaces in the spine results in compression of thenerve roots or spinal cord a condition known as spinal stenosis mayoccur. Spinal stenosis occurs when bony spurs or soft tissues, such asdiscs, impinge upon the spinal canal to compress the nerve roots orspinal cord. Spinal stenosis occurs most often in the lumbar spine, butalso occurs in the cervical spine and less often in the thoracic spine.It is frequently caused by degeneration of the discs between thevertebrae due to osteoarthritis. Rheumatoid arthritis usually affectspeople at an earlier age than osteoarthritis does and is associated withinflammation and enlargement of the soft tissues of the joints. Theportions of the vertebral column with the greatest mobility, i.e., thecervical spine, are often the portions most affected in people withrheumatoid arthritis. However, there are known non-arthritic causes ofspinal stenosis. Some non-arthritic causes of spinal stenosis includetumors of the spine, trauma, Paget's disease of bone, and fluorosis

Therapeutic procedures to alleviate pain are restore function aredescribed in a progression of treatment from spinal arthroplasty tospinal arthrodesis. As used herein, spinal arthroplasty encompassesoptions for treating disc degeneration when arthrodesis is deemed tooradical an intervention based on an assessment of the patient's age,degree of disc degeneration, and prognosis. Spinal arthrodesis, orfusion, involves a discectomy, i.e., surgical removal of the disc,followed by the subsequent immobilization of a spinal motion segment. Aspinal motion segment is generally comprised of two adjacent vertebralbodies separated axially by a spinal disc. The procedure of discectomyand “fusion” of the vertebral bodies results in the two vertebraeeffectively becoming one solid bone. Accordingly, the procedureterminates all motion at that joint in an attempt to eliminate or atleast ameliorate discogenic pain. The benefit of fusion is pain reliefand the down side is elimination of motion at the fused joint, which canhinder function. This surgical option is reserved for patients withadvanced disc degeneration.

Several companies are pursuing the development of prosthesis for thehuman spine, intended to partly or completely replace a physiologicaldisc. In individuals where the degree of degeneration has not progressedto destruction of the annulus, rather than a total artificial discreplacement, a preferred treatment option may be to replace or augmentthe nucleus pulposus. This augmentation may involve the deployment of aprosthetic disc nucleus. As noted previously, the normal nucleus iscontained within the space bounded by the bony vertebrae above and belowit and the annulus fibrosus, which circumferentially surrounds it. Inthis way the nucleus is completely encapsulated and sealed with the onlycommunication to the body being a fluid exchange that takes placethrough the bone interface with the vertebrae, known as the endplates.The hydroscopic material comprising the physiological nucleus has anaffinity for water which is sufficiently powerful to distract (i.e.,elevate or “inflate”) the intervertebral disc space, despite thesignificant physiological loads that are carried across the disc innormal activities. These forces, which range from about 0.4× to about1.8× body weight, generate local pressure well above normal bloodpressure, and the nucleus and inner annulus tissue are, in fact,effectively avascular. In essence, the existence of the nucleus, as acushion, and the annulus, as a flexible member, contributes to the rangeof motion in the normal disc.

As noted previously, some current devices are configured to form anartificial disc or an artificial nucleus. However, many of these devicesare susceptible to movement between the vertebral bodies. Further, theymay erode or degrade. In addition, they may extrude through the site ofimplantation in the annulus or otherwise migrate out of place. Some ofthese drawbacks relate to the fact that their deployment typicallyinvolves a virtually complete discectomy achieved by instrumentsintroduced laterally through the patient's body to the disc site andmanipulated to cut away or drill lateral holes through the disc andadjoining cortical bone. The endplates of the vertebral bodies, whichcomprise very hard cortical bone and help to give the vertebral bodiesneeded strength, are usually weakened or destroyed during the drilling.The vertebral endplates are special cartilage structures that surroundthe top and bottom of each vertebra and are in direct contact with thedisc. They are important to the nutrition of the disc because they allowthe passage of nutrients and water into the disc. If these structuresare injured, it can lead to deterioration of the disc and altered discfunction. Not only do the large laterally drilled hole or holescompromise the integrity of the vertebral bodies, but the spinal cordcan be injured if they are drilled too posteriorly.

Alternatively, current devices are sometimes deployed through asurgically created or enlarged hole in the annulus. The annulus fibrosisconsists of tough, thick collagen fibers. The collagen fibers whichcomprise the annulus fibrosis are arranged in concentric, alternatinglayers. Intra-layer orientation of these fibers is parallel, however,each alternating (i.e., interlayer) layers' collagen fibers are orientedobliquely (˜120′). This oblique orientation allows the annulus to resistforces in both vertical and horizontal directions. Axial compression ofa disc results in increased pressure in the disc space. This pressure istransferred to the annulus in the form of loads (stresses) perpendicularto the wall of the annulus. With applied stress, these fibrous layersare put in tension and the angle from horizontal decreases to betterresist the load, i.e., the annulus works to resist these perpendicularstresses by transferring the loads around the circumference of theannulus (Hoop Stress). Vertical tension resists bending and distraction(flexion and extension). Horizontal tension resists rotation and sliding(i.e., twisting). While the vertical components of the annulus' layersenable the disc to withstand forward and backward bending well, onlyhalf of the horizontal fibers of the annulus are engaged during arotational movement. In general, the disc is more susceptible to injuryduring a twisting motion, deriving its primary protection duringrotation from the posterior facet joints; however, this risk is evengreater if and when the annulus is compromised.

Moreover, annulus disruption will remain post-operatively, and present apathway for device extrusion and migration in addition to compromisingthe physiological biomechanics of the disc structure. Other devices, inan attempt to provide sufficient mechanical integrity to withstand thestresses to which they will be subjected, are configured to be so firm,stiff, and inflexible that they tend to erode the bone or becomeimbedded, over time, in the vertebral bodies, a phenomenon known as“subsidence”, sometimes also termed “telescoping”. The result ofsubsidence is that the effective length of the vertebral column isshortened, which can subsequently cause damage to the nerve root andnerves that pass between the two adjacent vertebrae.

SUMMARY OF THE INVENTION

In one aspect, the present invention may provide alternative options fortreating disc degeneration when arthrodesis or fusion is deemed tooradical an intervention based on an assessment of the patient's age,degree of disc degeneration, and prognosis. Specifically, the presentinvention includes an axially deployed mobility preservation apparatuswhich may provide discogenic pain relief and dynamic stabilization, byelevating and maintaining distraction while preserving mobility. Themobility preservation apparatus may also restore or manage range ofmotion and, thereby substantially improve biomechanical function ascompared to current methods and devices.

In the context of the present invention, “biomechanics” will refer tophysiological forces on intervertebral disc components (individually andcollectively) attributable to movement of the lumbar spine, described inthe previous explanation of the six degrees of freedom which comprisespinal range of motion.

In one aspect of the present invention, spinal axially deployed mobilitypreservation apparatus are deployed into the disc space 860 in aminimally traumatic fashion via a trans-sacral, axial approach ratherthan laterally through the annulus. The trans-sacral, axial approachpermits axial implantation without compromising it anatomically orfunctionally impairing its physiological load sharing by accessing thenucleus laterally through a hole or fissure in the annulus. Risks ofexpulsion or migration through an uncompromised annulus are inherentlyless, and are reduced even further by a mobility preservation apparatushaving at least one bone anchor in accordance with the presentinvention. A mobility preservation apparatus in accordance with thepresent invention typically includes one or more bone anchor to engageat least one vertebral body. The mobility preservation apparatus mayalso include an access tract plug to seal at least one axial accesstract into at least one vertebral body through which the mobilitypreservation apparatus may be deployed. Yet another advantage of themobility preservation apparatus in accordance with the present inventionis that their axial deployment in the spine may enable anchoring in thevertebral body which is readily accessible through axial access methodsand provides a relatively large bone for anchoring.

In the context of the present invention, “dynamic” refers to non-staticdevices with an inherent ability to allow mobility by enabling orfacilitating forces or load bearing that assist or substitute forphysiological structures that are otherwise compromised, weakened orabsent. The mobility preservation apparatus of the present invention mayprovide dynamic stabilization across a progression-of-treatmentinterventions for treating symptomatic discogenic pain, ranging fromtreatment in patients where little degeneration or collapse is evidentradio-graphically, to those for whom prosthetic nucleus devices or totaldisc replacements are indicated. For example, a mobility preservationapparatus having a prosthetic nucleus would be indicated in patientswith a greater degree of degeneration and loss of disc height but not tothe stage where advanced annular break-down is present. A mobilitypreservation apparatus having a prosthetic nucleus may go beyond dynamicstabilization by filling the de-nucleated space left by an aggressivenucleectomy with an appropriate material. Here, the goal may be torestore, as opposed to preserve, disc height and motion. A mobilitypreservation apparatus having a larger prosthetic nucleus may provide atotal disc replacement which would be indicated with more advanceddisease than with a standard prosthetic nucleus but where some annularfunction remains. The mobility preservation apparatus in accordance withthe present invention may be configured to augment, preserve, restore,and/or manage the physiological function according to the interventionindicated. In general, the axial mobility preservation apparatus of thepresent invention disclosed herein may be configured as devices with anaspect ratio of greater than 1. That is, the dimension in the alvertebral plane of the mobility preservation apparatus is greater thanits dimension in any orthogonal direction to that axial plane in closeproximity to the physiological instantaneous center of axial rotation.The mobility preservation apparatus can be deployed in an orientation inapproximately the line of principal compressive stress. Further, themobility preservation apparatus can be placed at approximately thecenter of rotation vis à vis a human disc motion segment.

In one aspect of the present invention, certain embodiments of themobility preservation apparatus include an elongated body having anintermediate section in between distal and proximal threaded bone anchorportions. The intermediate section may include at least a portion whichis flexible. The flexible portion may be a cable, a spring, a spiralflexure, a notched flexure, a flexible coupler, a set of stacked-washersover a cable or wire, inflatable bladder (e.g., expandable membrane), oranother element or combination of elements as will be recognized bythose skilled in the art upon review of the present disclosure. Theflexible portion may serve as the dampener previously described and isable to assimilate forces or redistribute loads. Hence, in accordancewith this aspect of the present invention, the mobility preservationapparatus may include a proximal and a distal body connected by anintermediate section with one or more flexible portions. The elongatedbody may be configured from a one-piece, two-piece, or three-piecedesign. The mobility preservation apparatus may be configured with orwithout an expandable membrane that may maximize surface area over whichloads are distributed, and that may or may not assist in distraction.The expandable membrane may be integral, it may be elastomeric orelastic, and it may be foldable to expand into an unfolded position tobe expandable. The mobility preservation apparatus may include cabledesigns for its intermediate section. The cable may be one piece offixed length with or without an inflatable membrane, or two or moreparts of variable length. The mobility preservation apparatus mayinclude ball and track multi-component designs as the flexible portionof the intermediate section. In another embodiment, an expandablemembrane component may be folded within a cannulated section of themobility device during device delivery to the target site, and thendeployed, e.g., unfolded, in situ via expansion by infusion orinflation. A non-inflatable collar may also be provided and used inconjunction with the expanding membrane. The collar is first deployed byfolding or may be secured to the expanding membrane, and is configuredto conform with (i.e., buttress) the annulus to prevent migration orleakage of the membrane through herniations. The cross sectional area ofthe collar is stiffer, relative to the expandable membrane. In apreferred embodiment the collar is from between about 8 mm-12 mm highand about 0.5 to 1.0 mm thick (see FIGS. 13 & 14). In anotherembodiment, the mobility preservation apparatus may include a singlebone anchor secured to a prosthetic nucleus.

In another embodiment of the present invention, the axially deployed,spinal mobility preservation apparatus is configured to further includea prosthetic nucleus including a semi-compliant expandable membrane tocontain at least one prosthetic nucleus material. The prosthetic nucleusmaterial is typically biocompatible and may be physiologic saline, anelastomer, a hydrogel, or combinations or blends thereof. In one aspect,the prosthetic nucleus is configured to functionally reproduce the sameload-bearing characteristics as the natural disc's nucleus pulposus, topreserve and restore mobility. The expandable membrane may be configuredto contain prosthetic nucleus material delivered through an elongateddelivery tube or catheter, wherein the distal end of the tube isinserted through a cannula into the proximal end of the mobilitypreservation apparatus which end engages an expandable membrane which isattached in or on the elongated body. The expandable membrane istypically attached or secured to the elongated body by an adhesive or bylaser-welding of a retainer ring either to the interior walls of a lumenat the proximal end of the elongated body, or directly to its distalend. The expandable membrane is enlarged or filled through a lumen,apertures or fenestrations which enable fluid communication between theexterior and interior of the intermediate segment of the mobilitypreservation apparatus and through which prosthetic nucleus material isinfused into the expandable membrane which is deployed over andsurrounding the intermediate segment. In this manner, the expandablemembrane may inflate and extend into the intervertebral disc space. Inone aspect, when filled, the expandable membrane conformably contactsthe surfaces within the intervertebral disc space. The expandablemembrane may be configured from an elastomeric material, with aDurometer Shore A hardness in the range of substantially about 20-90,that is deployable, stable and biocompatible in situ, e.g., such assilicone rubber. The ultimate expansion of the membrane is typicallylimited by contact with the end plates and the annulus, preventingrupture of the membrane due to over inflation.

The expandable membrane is typically expanded with a prosthetic nucleusmaterial that may include elastomeric solids and/or viscoelastic gels,i.e., materials whose viscoelastic properties (e.g., rheology andcompressibility) in conjunction with the biomechanical properties ofouter expandable membrane, enable them to perform in an functionalmanner which is substantially equivalent to the physiologic discnucleus. In one embodiment, the prosthetic nucleus material is ahydrogel. The hydrogel can be introduced into the membrane in a liquidor dry particulate form or in microspheres or beads. For example, onehydrogel is formulated as a mixture of hydrogel polyacrylonitrile or anyhydrophilic acrylate derivative with a unique multiblock copolymerstructure or any other hydrogel material having the ability to imbibeand expel fluids while maintaining its structure under various stresses.

As an example, the hydrogel can be formulated as a mixture of polyvinylalcohol and water. The hydrogel core formed within the envelope willswell as it absorbs fluids through the porous fabric wall of theenvelope, preferably in the manner of a native nucleus. When fullyhydrated, the hydrogel core may have a water content of between 25-95%.The hydrogel material of one suitable embodiment is manufactured underthe trade name Hypan® by Hymedix International, Inc., and has a watercontent of about 80-95%. Another hydrogel system comprises naturalhyaluronan gels or blends that may be chemically altered to enhancestructure, e.g., scaffolding ability or physical state, and optimizebiomechanical properties in situ via laser exposure, e.g., to convertliquid into a solid. Yet other hydrogels (Poly(ethylene glycol) (PEG);Poly(ethylene Oxide) (PEO); Poly(vinyl pyrollidine) (PVP)) or blends ofhydrogels (e.g., Poly(vinyl acetate) (PVA)/PVP; PEG-based/Polyethylene(PE) glycated hydrogels; crosslinked aliphatic polyoxaamide polymers; orcombinations of synthetic and native polypeptides or glycosaminoglycans(GAGs) such as such as actin, fibrinogen, collagen and elastin;chondroitin, keratin and dermatan sulfate; chitosan) and/or elastomersor other combinations (e.g., incorporation of an ionic or hydrophobicmonomer into the hydrogel network, to engineer a reversibly responsivepolymer) that optimize desired intramolecular and intermolecular bondingarrangements and reproduce the viscoelastic properties of the nativenucleus, may also be used. Conceptually, this is enabled fromunderstanding fundamental relationships between the structure of thepolymer (e.g., molecular weight; cross-linking density, etc.) underphysiological conditions and the physical properties of the resultinghydrogels. As noted previously, the native nucleus consists of mostlytype II collagen (cartilage like) and large protein macromoleculescalled proteoglycans that absorb water into the disc and are extremelyimportant to the biomechanical properties of the disc. Hydrogels modifytheir molecular arrangement, volume and/or phase when acted upon by aspecific stimulus such as temperature, light, a pH change or otherchemical inducement, osmotic pressure or mechanical stress, or electricfield, and selection of the prosthetic nucleus material of the presentinvention is not limited in scope with respect to materials' triggerstimuli, and may include those that are either chemically and/orphysically cross-linked gels (e.g., via ion-complexation or those thatare thermoreversibly cross-linked) as suitable. Hence in a preferredembodiment, the prosthetic nucleus material is selected based on itsstability under physiological conditions and/or in physiological fluids,including the ability withstand load, resist shear stresses and fatigueforces, or other factors that might otherwise induce fragmentation orotherwise promote extrusion or migration, or fractional mass loss overtime.

As noted earlier, the mobility preservation apparatus of the presentinvention may be deployed following complete or partial nucleectomy toremove all or most of the nucleus, respectively, to create a spacewithin the intervertebral disc. However, generally the access tractthrough which a prosthetic nucleus device containing the prostheticnucleus material is axially deployed will be smaller spatially than thevolume of the intervertebral disc space 860 to be augmented or replaced.To compensate for the spatial discrepancy, in one variation, alreadydescribed above, an expandable membrane or bladder is inserted in arelaxed, folded or unfilled state into the prepared space and theprosthetic nucleus material is injected or infused into the bladder, toexpand the membrane in situ.

In another aspect, the expandable membrane comprises a moisturepermeable expandable membrane. The expandable membrane is partiallyfilled with a hydroscopic prosthetic nucleus material prior to axialdeployment into the intervertebral disc space. Due to an infusion ofmoisture from physiologic fluid present in the disc space 860 throughthe moisture permeable expandable membrane, the prosthetic nucleusmaterial subsequently swells in situ, leaving an expanded,semi-compliant prosthetic nucleus that provides distraction and maximumsurface area in conformable contact with the intervertebral disc space860 surfaces and structures. In some embodiments, this prostheticnucleus may effectively distribute physiologic loads. In yet anothervariation, a partially filled permeable membrane may be pre-formed andsealed prior to deployment

In some aspects of the present invention, the mobility preservationincludes an expandable membrane configured to contain prosthetic nucleusmaterial comprising hydrogels that are implanted or inserted in adehydrated condition, for example by means of a glycerin carrier. Thehydrogels are typically 3-dimensional structures consisting mainly ofhydrophilic (i.e., very high affinity for water) polymeric materials orcopolymers which retain water, without dissolving, within a networkstability that is achieved through the presence of chemical or physicalcross-links (e.g., entanglements; crystallites; primary covalent orsecondary hydrogen, ionic, or Van der Waals bonds). In this manner, theoverall bulk of the mobility preservation apparatus is reduced, allowingit to be inserted through a smaller access and the subsequent hydration,which results in an increase in volume of the hydrogel, may assist indisc distraction to at least partially restore disc height. In certainembodiments the prosthetic nucleus material may remain fluid, while inother embodiments, devices may be configured for deployment via minimalaccess, by introducing the hydrogel or polymeric material in a firststate or condition (e.g., flowable), and to then allow or induceconversion of the material to a second phase or state, (e.g., to asolid). In this manner, the material can be introduced through thesmallest possible access and yet still be provided in sufficientquantity to fill the disc space 860 and provide the desired function.Examples of methods to convert a material from a first flowable state toa second solid state include but are not limited to: a temperature phasechange as from a melted state to a frozen state, polymerization of amonomer or low molecular weight polymer such as with the use of acatalyst; laser or UV cross-linking of a liquid polymer resulting in asolid; leaching of a solvent by replacing it with water (for example:polyacrylonitrile-polyacrylamide hydrogel can be dissolved indimethylsulfoxide (DMSO) resulting in a flowable liquid which willinstantly transform to a solid in the presence of water, into which theDMSO will preferentially flow); and use of reverse gelation polymers,such as Pluronic™, commercially available from BASF, Inc., Mount Olive,N.J. (USA), that are liquid at room temperature and form a solid atelevated temperatures such as body temperature, etc.

The specific configuration of the expandable membrane may vary dependingon the intended treatment or function. For example, in anotherembodiment of the present invention, the moisture-permeable expandablemembrane is configured from a bioabsorbable material which remains inplace to assume physiologic loads while the prosthetic nucleus materialis swelling and the access site is healing.

In yet another aspect of the present invention, the expandable membraneis coated on its external surface with a therapeutic agent to bedelivered, administered to the structures in which it is in conformalcontact when fully filled and deployed.

In another embodiment, the expandable membrane component containspre-inserted prosthetic nucleus material sealed therein. Morespecifically, the expandable membrane is configured as an envelopecomprising tightly woven or knit fabric of polymeric fibers, e.g.,Dacron™ or other material with a comparable tensile modulus. Theenvelope may be formed from other tightly woven, high molecular weight,high tenacity, flexible polymeric fabric e.g., high molecular weightpolyethylene, polyester, polyolefin, polyethylene terephthalate,polytetrafluoroethylene, polysulfone, nylons, or any other highmolecular weight, and other high tenacity materials including carbonfiber yarns, ceramic fibers, metallic fibers, etc. The fabric has poresthat are small enough to confine the prosthetic nucleus material withinthe envelope while allowing passage (e.g., bi-directional) of lowmolecular weight hydration fluids or therapeutic agents. Preferably, theopenings have an average diameter of approximately 10 micrometers,although other dimensions are acceptable. While the fabric is describedas woven, any other configuration having a semi-permeable or microporousattribute can be used. The flexible material allows expansion andcontraction of the hydrogel core (prosthetic nucleus material) in acontrolled fashion. The hydrogel core serves to cushion the disc in themanner of the native nucleus pulposus more effectively when the envelopefabric is flexible and semi-compliant, having a burst strength that isgreater than the swelling pressure of the hydrogel core when fullyhydrated to prevent rending and loss of the hydrogel core. By having anenvelope or jacket that is both flexible and semi-compliant, i.e., theelasticity being “modulus matched” to the native nucleus, the filledenvelope is effective as a dampener, and properly assumes anddistributes loads. When fully hydrated, an inelastic envelope expandedto its full capacity may not stretch or give when a load is applied andcannot deform, since the compression modulus in the normal load range isnearly vertical or incompressible, or distribute loads uniformly,thereby likely resulting in device subsidence and/or transitionsyndrome.

In another aspect, the expandable membrane comprises non-woven fibers,e.g., of a similar type used in the manufacture of Tyvek™ film.

In yet another aspect, a moisture-permeable polymeric expandablemembrane may comprise a biaxially oriented membrane modified to bemicroporous by mechanical means, such as of laser drilling, or bychemical means, e.g., leaching out sacrificial salt particles to achievea satisfactory end configuration.

In another aspect of the present invention, the expandable membranecontaining the prosthetic nucleus material is configured to include anintegral collar that extends axially about the expandable membrane andwhich is either secured to or integrally formed within the expandablemembrane. The integral collar makes the expandable membrane is stifferat its lateral surfaces than its inferior and superior surfaces. Theexpandable membrane with an integral collar may be deployed alone or incombination with a separate elastomeric collar that can abut theannulus. Mobility preservation apparatus with the integral collar mayeffectively functions as a prosthetic disc and provide the sameload-bearing characteristics as the natural disc without undergoing theinvasiveness of current total disc replacement procedures. The balanceof compliancy and stiffness whether provided as a collar and/or as anexpandable membrane with an integral stiffer component as previouslydescribed, enables the expanded expandable membrane to cushion a motionsegment, without collapsing under compressive load. That is, theexpanded expandable membrane with or without additional componentprovides dampening of instantaneous load, and also maintains effectivesurface area of conformal contact (congruent interface) between themobility preservation apparatus and the annulus and the inferior andsuperior end plate surfaces, respectively. In turn, this yields a moreuniform, radial distribution of loads to more closely approximatephysiological load sharing. In accordance with this aspect of thepresent invention, the mobility devices disclosed herein are less likelyto cause the phenomena of subsidence and/or transition syndrome. As usedherein, subsidence refers to the detrimental descent of an orthopedicimplant into bone that surrounds it. Transition syndrome refers toaltered biomechanics and kinematics of contiguous vertebral levels andconcomitant risk of adjacent motion segment instability that may occuras a result of spinal therapeutic procedures that are suboptimal interms of their ability to restore physiological function and properties,and thus risk a cascading deleterious effect on surrounding otherwisehealthy tissue.

In another variation of the present invention, the previously notedspatial discrepancy is obviated by prosthetic nucleus materials whichare deployed directly into the denucleated space to augment or replacethe intervertebral disc, after which the access tract is sealed in orderto retain the prosthetic nucleus material by precluding migration orexpulsion. More specifically, the prosthetic nucleus material,independent of an expandable membrane or envelope, may be introduceddirectly into the prepared intervertebral disc space, when the annulusis known to be intact. In this embodiment, the prosthetic nucleusmaterial serves as the “air” in the “tire” of the disc, and is selectedbased on, among other attributes, including: biocompatibility;resistance to fractional mass loss over time and hence ability toprovide or maintain distraction over time; distribute loads (hydrostaticpressure in the disc), for example. Further, the prosthetic nucleusmaterial may be particularly selected to restore tensile hoop stress inthe annulus. The selected prosthetic nucleus material may includebiomedical grade hydrogels or blends thereof (e.g., hydrogel/hydrogel,or hydrogel/elastomer). Cross-linked hyaluronic acid, such as isavailable from Fidia Corporation in Italy, is an example of a suitablematerial, however, many natural and man-made hydrogels or blends thereofmay be configured to achieve similar properties without inflammatoryresponse. The efficacy of this embodiment for its intended function isfurther predicated on the requirement that the access site to the discspace 860 is sealed to preclude prosthetic nucleus material migration orexpulsion. Any one of numerous valve configurations, e.g., self-sealingvalve assemblies or flow-stop devices may suitably serve this function.Materials suitable for anchoring are similarly suitable as plugs, suchas non-absorbable threaded plugs, including those fabricated frommedical grade polyether-ether-ketone (PEEK) such as that commerciallyavailable from Invibio Inc., in Lancashire, United Kingdom, orpolyether-ketone-ketone (PEKK) available from Coors-Tech Corporation, inColorado, or alternatively, conventional polymethylmethacrylate (PMMA);ultra high molecular weight polyethylene (UHMWPE), or other suitablepolymers in combination with autologous or allograft bone dowels may beused as plugs. One advantage of this embodiment would be the concomitantuse of prosthetic nucleus material comprising hydrogels as in situdelivery vehicles for a range of therapeutic compounds (e.g.,capitalizing on their biodegradability, through chemical hydrolysis oras enzymatically catalyzed). Moreover, certain bioactive hydrogels(e.g., such as those based on photo-crosslinked poly(ethylene oxide)[PEO], or block polypeptide or amino acid hydrogels), may be used toengineer tissue, e.g., cartilage; or to serve as a dimensional matrixthat promotes nerve regeneration, or reduces scar tissue formation.

In another aspect of the present invention, the mobility preservationapparatus may both decompress the disc and alleviate pain usually causedby posterior nerve impingement, by either inducing slight segmentalkyphosis or straight elevation; or by creating limits and/or resistanceto segmental motion. In this manner, mobility preservation apparatus areable to provide both stable anterior and posterior load support (e.g.,loads that may approximate 10 times the body weight of a patient) andadequate medial-lateral and rotational support, without adjunctiveposterior instrumentation and without accompanying osteogenesis. Thus,unlike spinal fusion devices where accompanying osteogenesis results inpermanent immobilization of the motion segment, the mobilitypreservation apparatus of the present invention may be revisable, andalso convertible. That is, they may be explanted, replaced, or convertedin the progression-of-treatment options from prosthetic nucleus to totaldisc replacement up to and including fusion. In certain embodiments ofthe present invention, the mobility preservation apparatus may includemultiple, modular components which facilitate such convertibility. Forexample, a rod or plug may be inserted into the proximal end of acannulated female inferior anchor portion. The plug may extendsufficiently through and distally into the fenestrated midsection of theelongated that the effective flexibility of the mobility preservationapparatus is compromised. If compromised sufficiently, the mobilitypreservation apparatus with the plug may now serve as an immobilizationor fusion device. In addition, osteogenic materials may also beintroduced through the into the intervertebral disc space 860 tofacilitate the fusion of adjacent vertebrae.

In a yet another aspect of the present invention, the mobilitypreservation apparatus of the present invention is configured to includebiocompatible materials that meet ISO 10993 standards for long-termimplants, and/or are able to withstand, without wear, long term normalranges of physiological loading. For example, the mobility preservationapparatus could withstand between about 1250 Newtons (N) (280 lbf) and2250 N (500 lbf) axial compression; 100 N (25 lbf) and 450N (100 lbf) ofboth lateral and sagittal shear, respectively, through full ROM over thelifetime of the mobility preservation apparatus, or up to about 40×10⁶cycles. Additionally, the mobility preservation apparatus of the presentinvention are preferably able to tolerate short term maximumphysiological loads through full ROM of about 8000 Newtons (N) (1800lbf) axial compression; about 2000 N (450 lbf) lateral shear; and about3000 N (675 lbf) sagittal shear, over about 20 continuous cycles withoutfailing. Yet another advantage of the mobility preservation apparatus ofthe present invention, in addition to sharing and distributingphysiological loading and motion, is the ability to also assimilateforces artificially introduced in the event of ancillary therapeuticprocedures, such as for example pedicle screw insertion.

In the context herein, “biocompatible” refers to an absence of chronicinflammation response when or if physiological tissues are in contactwith, or exposed to (e.g., wear debris) the materials and devices of thepresent invention. In addition to biocompatibility, in another aspect ofthe present invention the materials of the mobility preservationapparatus are sterilizable; visible and/or imageable, e.g.,fluoroscopically; or via CT (computed tomography), or MRI (magneticresonance imaging). It will be understood that with MRIs, the materialsmust be substantially free of iron. Moreover, in consideration ofcontrast, detail, and spatial sensitivity, it is contemplated thatcontrast media or other materials (e.g., barium sulfate) may be employedin configuring mobility preservation apparatus when and where needed andappropriate, to supplement or modify radiolucency or radio-opaqueness.

It is another aspect of the present invention to provide spinal mobilitypreservation apparatus which preferably do not impede the mobility of,and are responsive to the physiological Instantaneous Center of Rotation(ICOR). Further, in one embodiment, the mobility preservation apparatusprovides anterior-posterior translation and has a mobile ICOR. Themobility preservation apparatus of the present invention may notadversely impact the stiffness of the motion segment being treated. Forexample, mobility preservation apparatus axially deployed in the L5-S1lumbar spine may enable/accommodate a range of motion of between about10° to 15° flexion; between about 7° to about 10° extension; about 4° toabout 9° of left or right lateral bending and between about 1° to about2° clockwise or counterclockwise axial rotation. Mobility preservationapparatus implanted in L4-L5 may enable/accommodate a range of motion ofbetween about 8° to 10° flexion; between about 5° to about 7° extension;between about 4° to about 9° left or right lateral bending; and betweenabout 1° to about 4° clockwise or counterclockwise axial rotation.

In yet another aspect of the present invention, mobility preservationapparatus may be configured to manage mobility by controlling resistanceto motion. This can be accomplished by varying stiffness which may beaccomplished by varying cross-sectional area and other features, andhence stiffness. In addition or alternatively, mobility preservationapparatus may be configured to create limitations to motion. This can beaccomplished by incorporation of at least one mechanical stop. As usedherein, “resistance” refers to the force required to move through a fullrange of motion, whereas in contrast, “limitation” refers to not forcebut degree, i.e., curtailment of full range of motion in one or moredirections.

Thus, in one aspect of the present invention, mobility preservationapparatus may be configured to restore full and/or unconstrained rangeof motion, but in general the expanded expandable membranes function torestore mobility rather than control or manage motion. In contrast, yetanother aspect of the present invention includes mobility preservationapparatus which provide motion management. The mobility preservationapparatus which provide motion management may allow a semi-constrainedrange of motion where full range of motion is allowed in combinationwith increased resistance to motion; or a limited range of motionwherein the extent of motion in one or more degrees of freedom ismechanically limited, with or without increased resistance to motion.The mobility preservation apparatus which provide motion management maybe configured to include expandable membranes, and hence incorporate themechanical functions of a prosthetic nucleus material.

Yet another aspect of the present invention to provide spinal mobilitypreservation apparatus that induces and maintains maximum distractionwhile being implantable and functional within a wide range in anatomies.In one embodiment, mobility preservation apparatus provide from betweenabout 2 mm to about 10 mm, of distraction, and can accommodatephysiological lateral disc diameter from between about 15 mm up to about50 mm; sagittal disc diameter from between about 10 mm up to about 40 mm(i.e., in the median plane between the anterior and posterior sides);disc heights from between about 5 mm and about 15 mm; and “wedge angles”from between about 5 degrees and about 15 degrees. As used herein, wedgeangle refers to the relative angle of the faces of the inferior andsuperior vertebral endplates of a motion segment, one to the other.

In certain aspects, the present invention may involve surgical toolssets and methods for accessing and preparing vertebral elements, such asinter-vertebral motion segments located within a human lumbar and sacralspine for therapeutic procedures. In the context of the presentinvention, “motion segments” comprise adjacent vertebrae separated byintact or damaged spinal discs.

In particular embodiments of the present invention, instrumentationsystem components and their methods of use, individually and incombination and over or through one another, form or enlarge a posterioror anterior percutaneous tract; access, fragment and extract tissue(e.g., nucleus pulposus,); or otherwise prepare vertebral elements andinter-vertebral motion segments for dynamic stabilization viaimplantation of therapeutic procedures and spinal devices.Instrumentation may be introduced and aligned through the percutaneouspathways and according to the trans-sacral axial access methods. Thealignment may be accomplished using biplane fluoroscopy, endoscopy, orother radio-imaging methods, as guidance to insure that the channel ispositioned mid-line to the anterior/posterior and lateral sacral view.

Certain of the surgical tools take the form of elongated solid bodymembers extending from proximal to distal ends thereof. Such solid bodymembers may be used in combination or sequentially with elongated,cannulated body members. Hence, for example, design constraints, inaddition to outside diameter (O.D.) tolerances and limitations imposedby virtue of patient anatomies, such as tube wall thickness, materialselection/mechanical strength, and inside diameter (I.D.) also becomeconsiderations, e.g., to enable unrestricted passage over guide membersor through hollow body members without incurring deformation that mayimpair or otherwise preclude intended function. Certain of these solidbody and hollow body members can have distal means, mechanisms, orapertures that may be configured or manipulated for either precluding orfacilitating engagement with tissue; the latter including piercing;tapping; dilating; excising; fragmenting; extracting; drilling;distracting (e.g. elevating); repairing; restoring; augmenting; tamping;anchoring; stabilizing; fixing, or fusing tissue. Certain of these solidbody and hollow body members can have proximal means, mechanisms, pins,slots or apertures that may be configured or manipulated to engage;grasp; twist; pilot; angle; align; extend; expose, retract; drive;attach or otherwise interact to enable or facilitate the functionalityof other components within the surgical tools set, e.g., the distalmeans and mechanisms noted above in this paragraph. Moreover, inaccordance with the present invention the individual componentscomprised in the tools sets, or kits, may include a guide pinintroducer; guide pins with various distal end and proximal endconfigurations (e.g., tips; handles, respectively); soft tissue and bonedilators and dilator sheath(s); cutters; tissue extraction tools; twistdrills; an exchange rod and exchange cannula system; distraction tools;augmentation, and repair tools.

In a particularly preferred procedure, these instrumentation systemcomponents are visualization aligned axially, and progressively insertedinto a human lumbar-sacral spine through the minimally invasivepercutaneous entry site adjacent the coccyx to access the S1-L5 or L4disc space 860 to perform a partial or total nucleectomy, withoutcompromising the annulus fibrosis, unlike current surgical discectomyprocedures. Conventional discectomies are performed through a surgicallycreated or enlarged hole in the annulus that remains post-operatively,and represents a contraindicated pathway for extrusion and migration ofnatural or augmented tissue, or implants, and that also compromise thebiomechanics of the physiological disc structure.

Moreover, in accordance with the techniques and surgical tool sets, andin particular the cutters and extraction tool configurations of thepresent invention, a substantially greater amount (volume) of nucleuspulposus by, in comparison with other non-open discectomy procedures inpractice, may be removed, as needed. In particular, the instrumentationsystems and techniques embodied in the present invention moreeffectively, with less immediate trauma, and without residual negativephysiological impacts that may occur as a result of invasion of theannulus, prepare an inter-vertebral motion segment for subsequentreceipt of therapeutic procedures, and enables axial placement ofimplants close to and in alignment with the human spine's physiologicalcenter of rotation.

Other specific advantages over current practice may include: the patientis in a prone position that is easily adaptable to other posteriorinstrumentation; blood loss is minimal; soft tissue structures, such asveins, arteries, nerves may be are preserved; substantially lesssurgical & anesthesia time is required compared with conventionalprocedures; and the implants of the present invention may restorefunction not merely alleviate pain. These and other advantages andfeatures of the surgical tools sets and techniques disclosed in thepresent invention will be more readily understood from the followingdetailed description of the preferred embodiments thereof, whenconsidered in conjunction with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a perspective view of an embodiment of a mobilitypreservation apparatus without an expandable membrane;

FIG. 2 illustrates a cross-section of a side view of an embodiment of amobility preservation apparatus without an expandable membrane having aunitary elongated body and a lumen extending over the entire length ofthe elongated body;

FIG. 3 illustrates a cross-section of a side view of an embodiment of amobility preservation apparatus without an expandable membrane having amale portion and a female portion and a cavity which terminates prior tothe distal end of the distal body;

FIG. 4 illustrates a proximal end view of an embodiment of a mobilitypreservation apparatus;

FIG. 5 illustrates a perspective view of an embodiment of a mobilitypreservation apparatus with an expandable membrane in the expandedposition;

FIG. 6 illustrates a cross-section of a side view of an embodiment of amobility preservation apparatus having a unitary elongated body with anexpandable membrane in an unexpanded position;

FIG. 7 illustrates a cross-section of a side view of an embodiment of amobility preservation apparatus having a unitary elongated body with anexpandable membrane in an expanded position;

FIG. 8 illustrates a cross-section of a side view of an embodiment of amobility preservation apparatus having a multi-piece configuration witha male portion and a female portion with an expandable membrane in anunexpanded position;

FIG. 9 illustrates a cross-section of a side view of an embodiment of amobility preservation apparatus having an expandable membrane connectingthe proximal body and the distal body in an unexpanded position;

FIG. 10 illustrates a cross-section of a side view of an embodiment of amobility preservation apparatus having an expandable membrane connectingthe proximal body and the distal body in an expanded position and acentral lumen which terminates prior to the distal end of the distalbody;

FIG. 11 illustrates a perspective view of an embodiment of a mobilitypreservation apparatus with the expandable membrane in an expandedposition;

FIG. 12 illustrates a cross-section of a side view of an embodiment of amobility preservation apparatus with the expandable membrane in anunexpanded position;

FIG. 13 illustrates a cross-section of a side view of a single anchorprosthetic nucleus apparatus with the expandable membrane in theexpanded position;

FIG. 14 illustrates a cross-section of a side view of an embodiment of amobility preservation apparatus with a cable design for the flexibleelement;

FIG. 15 illustrates a cross-section of a side view of an embodiment of amobility preservation apparatus with a stacked washer design for theflexible element;

FIG. 16 illustrates a cross-section of a side view of an embodiment of amobility preservation apparatus with a ball and track configuration forthe flexible element;

FIG. 16 b illustrates a perspective view of an embodiment of a proximalinsert from a mobility preservation apparatus with a ball and trackconfiguration;

FIG. 17 illustrates a perspective view of an embodiment of a plug inaccordance with the present invention;

FIG. 18 illustrates a cross-section of a side view of an embodiment of amobility preservation apparatus in accordance with the present inventionincluding a plug within the proximal lumen and intermediate lumen;

FIG. 19 illustrates a perspective view of a collar in accordance withthe present invention;

FIG. 20 illustrates a cross-section of a side view of a collar securedto an expandable membrane;

FIG. 21 illustrates a cross-section of an end view through theintermediate body of an embodiment of the expandable membrane as afolded envelope;

FIG. 22 illustrates a partial cross-section of an embodiment of aproximal body of a mobility preservation apparatus in accordance withthe present invention receiving a driver;

FIG. 23 illustrates a partial cross-section of an embodiment of amobility preservation apparatus in accordance with the present inventionwith a driver positioned to axially rotate the mobility preservationapparatus with the illustrated driver incorporating an introducer toinfuse prosthetic nucleus material into the intervertebral disc space;

FIG. 24 illustrates a partial cross-section of an embodiment of theproximal body of a mobility preservation apparatus in accordance withthe present invention with a driver positioned to axially rotate themobility preservation apparatus being inserted into a bore within avertebral body;

FIG. 25 illustrates a partial cross-section of an embodiment of amobility preservation apparatus in accordance with the present inventionaxially positioned between vertebral bodies of a spinal motion segment;

FIG. 26 illustrates a partial cross-section of an embodiment of amobility preservation apparatus in accordance with the present inventionaxially positioned between vertebral bodies of a spinal motion segmentafter prosthetic nucleus material provided through the introducer has atleast partially expanded the expandable membrane;

FIG. 27 illustrates a partial cross-section of an embodiment of amobility preservation apparatus in accordance with the present inventionaxially positioned between vertebral bodies of a spinal motion segmentafter prosthetic nucleus material has expanded the expandable membrane;

FIG. 28 illustrates a partial cross-section of an embodiment of amobility preservation apparatus in accordance with the present inventionwithout an expandable membrane which is axially positioned betweenvertebral bodies of a spinal motion segment after prosthetic nucleusmaterial has substantially filled the intervertebral disc space;

FIG. 28A illustrates a partial cross-section of an embodiment of amobility preservation apparatus in accordance with the present inventionwithout an expandable membrane which is axially positioned betweenvertebral bodies of a spinal motion segment after prosthetic nucleusmaterial has substantially filled the intervertebral disc space andafter insertion of the stiffening member;

FIG. 29 illustrates a cross-section of a side view of an embodiment of amobility preservation apparatus having single body positioned within avertebral body and with the expandable membrane expanded within theintervertebral disc space;

FIG. 30 illustrates a partial cross-section of an embodiment of amobility preservation apparatus having an embodiment of a preformedprosthetic nucleus in accordance with the present invention beingpositioned through the mobility preservation apparatus and into anintervertebral disc space;

FIG. 31 illustrates top view of an embodiment of a preformed prostheticnucleus in accordance with the present invention;

FIG. 32 illustrates a top view of a cross-section of an embodiment of amobility preservation apparatus in accordance with the present inventionaxially positioned between vertebral bodies of a spinal motion segment;

FIG. 33 illustrates a partial cross-section of an embodiment of amobility preservation apparatus in accordance with the present inventionaxially positioned between vertebral bodies of a spinal motion segmentwith an unexpanded the expandable membrane; and

FIG. 34 illustrates a partial cross-section of an embodiment of amobility preservation apparatus in accordance with the present inventionaxially positioned between vertebral bodies of a spinal motion segmentafter prosthetic nucleus material has at least partly expanded theexpandable membrane.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of mobility preservation apparatus 10 in accordance with thepresent invention are generally illustrated throughout the figures forexemplary purposes. A mobility preservation apparatus 10 are generallyconfigured as an elongated body that is substantially radiallysymmetrical about its longitudinal axis as illustrated throughout thefigures. A mobility preservation apparatus 10 may include a proximalbody 12, an intermediate body 14, a distal body 16 and an expandablemembrane 18 operatively connected to one another as discussed below. Inaccordance with the present invention, mobility preservation apparatus10 are generally configured to be implanted axially within the spine.More particularly, mobility preservation apparatus 10 may position atleast a portion of the mobility preservation apparatus 10 in the discspace 860 between two adjacent vertebrae when positioned within apatient. The mobility preservation apparatus 10 may carry at least aportion of the load which would normally be transferred to a healthyintervertebral disk. In one aspect, the adjacent vertebrae may be S1 andL5.

Various configurations and embodiments of mobility preservationapparatus 10 are illustrated throughout the figures. A mobilitypreservation apparatus 10 are typically formed or configured as a one,two, or three piece elongated body or may be otherwise configured aswill be apparent to those skilled in the art upon review of the presentdisclosure. The mobility preservation apparatus 10 may be configured asan elongated body that is substantially radially symmetrical about itslongitudinal axis. In one embodiment, a mobility preservation apparatus10 may include a body 12 secured to an expandable membrane 18. Inanother embodiment, a mobility preservation apparatus 10 may include aproximal body 12 and a distal body 16 secured to an expandable membrane18. In yet another embodiment, the mobility preservation apparatus 10may include a proximal body 12 and distal body 16 with the proximal body12 and distal body 16 secured to one another by an intermediate body 14with at least a portion of the intermediate body including a flexibleelement 22. In still another embodiment, the mobility preservationapparatus 10 may include a proximal body 12 and distal body 16 with theproximal body 12 and distal body 16 secured to one another by anintermediate body 14 and the intermediate body including an expandablemembrane 18.

As illustrated for exemplary purposes in FIGS. 24 to 30, 32 33 and 34,mobility preservation apparatus 10 may be secured to and extend betweenadjacent caudad and cephalad positioned vertebral bodies. The mobilitypreservation apparatus 10 is typically positioned at the approximatecenter of movement between the two vertebral bodies. The mobilitypreservation apparatus 10 may be generally configured for axialimplantation through an axial bore 850 in a vertebral body and at leastpartially into or through a disc space 860 resulting from thenucleectomy procedure. The implantation of embodiments of a mobilitypreservation apparatus 10 may be enabled following partial or completenucleectomy and temporary distraction. As used herein, temporarydistraction refers to the elevation of disc height relative to anadjacent disk. A mobility preservation apparatus 10 may be inserted intoan elevated disc space 860 or may itself elevate the disc space 860 and,thereafter, may preserve that height space. Suitable surgicalinstrumentation systems and techniques for access, distraction andimplantation using an anterior trans-sacral axial approach are disclosedin commonly assigned U.S. patent applications having Ser. Nos.10/309,416; 10/125,771; 09/848,556; and 60/182,748, and U.S. Pat. Nos.6,558,386; 6,558,390; 6,575,979; and 6,790,210, the disclosures of whichare hereby incorporated by reference. Those skilled in the art willrecognize additional methods for implantation of a mobility preservationapparatus 10 upon review of the following disclosure and the attachedfigures.

A mobility preservation apparatus 10 implanted by the above referencedmethods may decompress the disc and alleviate pain caused by nerveimpingement, usually posterior, either inducing slight segmentalkyphosis or straight elevation, and/or by creating limits and resistanceto segmental motion. In this manner, mobility preservation apparatus 10may be able to provide both stable anterior and posterior load supportand a degree of medial-lateral and rotational support, withoutadjunctive posterior instrumentation and without accompanyingosteogenesis. In some exemplary embodiments, a mobility preservationapparatus 10 may be configured to support loads that may approximate ten(10) times the body weight of a patient.

A mobility preservation apparatus 10 may be generally configured toprovide some degree of dynamic stabilization. In the context of thepresent invention, dynamic refers to non-static devices with an inherentability to allow mobility by enabling or facilitating forces or loadbearing that assist or substitute for physiological structures that areotherwise compromised, weakened or absent. More specifically, certainembodiments of mobility preservation apparatus 10 are disclosed whicheschew immobilization in favor of providing dynamic stabilization. Bydesign, some of these embodiments may resist and limit of motion in acontrolled manner. As used in this context, “resist” or “resistance”refers to the force required to move through a full range of motion,whereas in contrast, “limit” or “limitation” refers to not force butdegree. That is, “limit” or “limitation” refers to curtailment of fullrange of motion in one or more directions. With respect to the lumbarspine, the normal full range of motion typically allows for about twelve(12) degrees of flexion, about eight (8) degrees of extension, aboutnine (9) degrees of left or right lateral bend, and about 2 degrees ofclockwise or counterclockwise motion. Thus, mobility preservationapparatus 10 may be configured to include, for example, a mechanicalstop(s) to limit motion, and/or provide resistance to motion beyond adesired range.

Thus, for example, mobility preservation apparatus in accordance withthe present invention which have been configured for the lumbar spinegenerally can permit at least about four (4) degrees, often at leastabout eight (8) degrees and preferably no more than about twelve (12)degrees or fourteen (14) degrees of flexion. The device will generallypermit at least about four (4) degrees, often at least about six (6)degrees and preferably no more than about eight (8) degrees or ten (10)degrees of extension. The device will generally permit at least about0.5 degrees, often at least about one (1) degree and preferably no morethan about two (2) degrees or three (3) degrees of rotation, dependingupon the desired clinical performance. With respect to the spinegenerally, mobility preservation apparatus 10 in accordance with theinvention may provide up to about 100% or 125% of the normal full rangeof motion for any particular motion segment being treated. Endpoints tothe range of motion may either be abrupt, such as by the use of firstand second complementary mechanical stop surfaces, or may be dampened byan increase in resistance to further motion.

In one aspect, resistance to motion may be altered by varyingcross-sectional area and hence stiffness, any of a variety of gradualramps, friction surfaces or gradual dimensional mismatches, or otherbiomechanical properties relevant to preserving or restoringphysiological function with respect to mobility. In various embodiments,the mobility preservation apparatus 10 may be designed to provide full,unconstrained (e.g., beyond the normal anatomical) range of motion,semi-constrained (e.g., approximately normal anatomical) range of motionwhere full range of motion is allowed in combination with increasedresistance to motion, or limited range of motion wherein the extent ofmotion in one or more degrees of freedom is mechanically limited, withor without increased resistance to motion.

With particular respect to a mobility preservation apparatus 10incorporating the ball and track configuration as a flexible element 22,discussed below, the mobility preservation apparatus 10 may beconfigured for the lumbar spine generally will permit at least about 4degrees, often at least about 8 degrees and preferably no more thanabout 20 degrees of flexion (bending forwards). The mobilitypreservation apparatus 10 will generally permit at least about 4degrees, often at least about 6 degrees and preferably no more thanabout 20 degrees of extension (bending backwards). Rotation isunconstrained through normal range of motion.

In one embodiment, the mobility preservation apparatus 10 mayapproximate the biomechanical properties of the physiological vertebralor disc structure(s) depending on the particular function(s) for whichtherapeutic procedure(s) are indicated. In one aspect, the mobilitypreservation apparatus 10 may be substantially matched bulk andcompression modulus. As used herein, this is referred to as modulusmatched. Further, a series of mobility preservation apparatus 10providing dynamic stabilization may be provided across aprogression-of-treatment for treating symptomatic discogenic pain,ranging from treatment in patients where little degeneration or collapseis evident radio-graphically, or other conditions for which additionalspinal support may be beneficial.

In another aspect, a mobility preservation apparatus 10 is configured tomechanically and adjustably, distract a disc space 860 and is configuredto subsequently be deployed so that it is oriented in approximately theline of principal compressive stress. For example, the mobilitypreservation apparatus 10 may be configured to be placed atapproximately the center of rotation in a human disc motion segment.Moreover, the mobility preservation apparatus 10 may configured to bedeployable without compromising the annulus and to dissipate and sharephysiologic loads with the normal load bearing biological structures.These load bearing structures will typically include the annulus and thenucleus pulposus, either alone or in combination.

In an exemplary configuration having at least a proximal body 12 and adistal body 16, the overall length of mobility preservation apparatus 10may range from about 40 mm to about 60 mm. Typically, the mobilitypreservation apparatus 10 will have an aspect ratio of at least one.Typically, the proximal body 12 and distal body 16 will be substantiallycoaxially aligned to facilitate the axial implantation of mobilitypreservation apparatus 10 through a bore 850 in a vertebral body. Theaspect ratio measured as the ratio of the overall length to the largestdiameter along the length. Proximal body 12 and distal body 16 aretypically sized and configured based on size, weight and support needsfor a particular patient as will be recognized by those skilled in theart upon review of the present disclosure. In some exemplary embodimentsfor threaded bone anchors, a smaller mobility preservation apparatus mayinclude a proximal body 12 having first threads 32 with a major threaddiameter of about 0.5″ and a minor thread diameter of about 0.4″, and adistal body 16 having second threads 36 with a major thread diameter ofabout 0.4″ and a minor thread diameter of about 0.2″. A larger mobilitypreservation apparatus 10 may include a proximal body 12 having firstthreads 32 with a major thread diameter of about 0.6″ and a minor threaddiameter of about 0.4″, and a distal body 16 having second threads 36with a major thread diameter of about 0.43″ and a minor thread diameterof about 0.3″. Frequently, the major thread diameters range from about0.25″ to about 0.75″, and minor thread diameters range from about 0.125″to about 0.5″. However, the sizes and relative proportions may be variedas required for particular patients and applications.

The proximal body 12, intermediate body 14, and distal body 16 can beformed from a wide range of materials that will be recognized by thoseskilled in the art upon review of the present disclosure. In one aspect,the material may be a metal alloy such as, Ti6Al4V, Elgiloy™ (a superalloy of cobalt chrome), MP35N, stainless steel, or other materialsaccording to the biomechanical properties being selected by design.Typically, the alloy will be selected for high tensile strength, whichmay be greater than 300,000 pounds per square inch, as well as highfatigue strength. The components may also be formed from a biocompatiblematerial selected for its abrasion and/or wear resistant, such as forexample cobalt chrome superalloys, such as Stellite™, available fromDeloro Stellite, Inc., distributed from Goshen, Ind. In another aspect,the mobility preservation apparatus 10 may be configured from abiocompatible polymeric material, such as for example,polyether-ether-ketone (PEEK) or polyether-ketone-ketone (PEKK). Inother aspects, the mobility preservation apparatus may be constructedfrom polymers, metals, and other materials or combinations of materialswith suitable biomechanical properties to engage the vertebral body andwithstand the loads transferred to the mobility preservation apparatus10.

In embodiments including a proximal body 12 and distal body 16, themobility preservation apparatus 10 is generally configured to extendbetween adjacent cephalad and caudad vertebral bodies, such as forexample between the vertebral bodies of the L5 and S1 vertebrae.Exemplary positioning of such embodiments is illustrated in FIGS. 25 to28, 30, and 32 to 34. Proximal body 12 is generally configured to securethe proximal end of mobility preservation apparatus 10 to a caudadpositioned vertebral body 800. Accordingly, the proximal body 12 willtypically include a first retention structure 32. For exemplarypurposes, first retention structure 32 has been illustrated as a firstthread 32 formed on the exterior surface of the proximal body 12throughout the figures. The first retention structure 32 is configuredto secure proximal body 12 in a bore 850 through a caudad vertebral body800, as illustrated in FIGS. 26 and 27. In another aspect, firstretention structure 32 of proximal body 12 may be configured to becompressionally or slidably secure proximal body 12 within bore 850. Inyet another aspect, the first retention structure 32 may include aningrowth structure to promote bone ingrowth to secure proximal body 12within bore 850. Additional configurations for first retention structure32 will be evident to those skilled in the art upon review of thepresent disclosure. The proximal body 12 may define a proximal lumen 42.In certain embodiments, the proximal lumen 42 may function as accessport and may reside along the longitudinal axis. The proximal lumen 42may extend coaxially from the proximal end to the distal end of proximalbody 12.

Distal body 16 is generally configured to secure the distal end ofmobility preservation apparatus 10 to a cephalad positioned vertebralbody 900. Accordingly, the distal body 16 will typically include asecond retention structure 36. For exemplary purposes, second retentionstructure 36 has been illustrated as a second thread 36 formed on theexterior surface of the distal body 16 throughout the figures. Thesecond retention structure 36 is configured to secure distal body 16 ina bore 850 through a cephalad vertebral body 900, as illustrated inFIGS. 26 and 27. In another aspect, second retention structure 36 ofdistal body 16 may be configured to compressionally or slidably securedistal body 16 within bore 850. In yet another aspect, the secondretention structure 36 may include structure to promote bone ingrowth tosecure distal body 16 within bore 850. Additional configurations forsecond retention structure 36 will be evident to those skilled in theart upon review of the present disclosure. The distal body 16 may definea proximal lumen 42. The proximal lumen 42 may extend coaxially from theproximal end to the distal end of distal body 16.

In some embodiments, the intermediate body 14 is an intermediate portionthat bridges the proximal body 12 and distal body 16. The intermediatebody 14 may be configured to assimilate forces or redistribute loadsincurred between adjacent vertebrae. The intermediate body 14 istypically secured at a proximal end to the proximal body 12 and at adistal end to the distal body 16. In one aspect, the proximal body 12,the intermediate section 14 and the distal body 16 may be formed from asingle piece of material. The intermediate body 14 may include anintermediate lumen 44. Intermediate lumen 44 may extend from theproximal end to the distal end of the intermediate body 14 or may extendto the proximal end of the intermediate body 14 and be in fluidcommunication with one or more fenestrations 74, discussed below. In oneaspect, the intermediate lumen 44 may extend coaxially from the proximalend to the distal end of intermediate body 14.

The intermediate body 14 typically includes a flexible element 22 topermit a limited degree of movement between the relative positions ofthe proximal body 12 and the distal body 16. The flexible element 22 cancomprise at least a portion of the intermediate body 14 and may becoextensive with the intermediate body 14. The flexible element 22 ofintermediate body 14 may include a cable, a spring, a helical flexure, anotched flexure, stacked-washers, a ball and track configuration, or acombination thereof, or other flexible members to serve as a “shockabsorber,” a point of articulation and/or a pivot point for movement ofadjacent vertebrae secured to the mobility preservation apparatus 10.When the flexible element 22 of intermediate body 14 is a spring orhelical flexure, the helical flexure is typically configured to supporta static load of between 10 lbs and 300 lbs at 1 mm of deflection.Typically, the helical flexure is configured to support a static load ofbetween 50 lbs and 200 lbs at 1 mm of deflection. In an exemplaryembodiment, the helical flexure is configured to support about 100 lbs.at 1 mm deflection. The load supported by a helical flexure may bemodified, by changing the number of coils per turn, changing the springconstant, and/or by altering “waist” diameter of the helical flexure.

Further, the intermediate body 14 may define one or more fenestrations74 through which a prosthetic nucleus material 60 may be inserted intoan expansion chamber 20 defined by the expandable membrane 18. In otheraspects, the fenestrations 74 may relate solely to the function offlexible element 22. As discussed above, fenestrations 74 may be influid communication with an intermediate lumen 44 defined byintermediate body 14. The intermediate lumen 44 may extend coaxiallyfrom the proximal end of the intermediate body 14 to the fenestrations74. In other embodiments, intermediate lumen 44 extends from theproximal end to the distal end of intermediate body 14. Further,intermediate lumen 44 may be coaxial and in communication with theproximal lumen 42 when intermediate section 14 is secured to proximalbody 12.

The expandable membrane 18 is generally configured to expand into thedisc space 860 to provide a degree of support to adjacent vertebralbodies 800, 900. The expandable membrane 18 may define an expansionchamber 20. In one aspect, the expansion chamber 20 may be defined withthe expandable membrane 18 in a relaxed position. In other aspects, theexpansion chamber 20 is only defined when the expandable membrane 18 isat least in part expanded. The expandable membrane 18 is typicallyexpanded in situ by the addition and/or hydration of a prostheticnucleus material 60 into and/or in an expansion chamber 20 asillustrated in FIGS. 26 and 27. The expandable membrane 18 may maintainthe position of the prosthetic nucleus material 60 within the expansionchamber 20. The expandable membrane 18 may be configured to assimilateforces or redistribute loads incurred between adjacent vertebrae when inan expanded configuration. The expandable membrane 18 is typicallyconfigured to circumferentially extend about or from the longitudinalaxis of at least a portion of the proximal body 12, the intermediatesection 14 and/or the distal body 16. In one aspect, the expandablemembrane 18 may be configured to inflate within the disc space 860 untilit comformably contacts all or substantially all of the surfaces withinthe disc space 860. In the expanded position, expandable membrane 18 maybe configured to functionally reproduce the same load-bearingcharacteristics as the natural disc's nucleus pulposus. In this manner,a mobility preservation apparatus 10 may preserve and restore mobilityto a spinal motion segment. In embodiments with an intermediate body 14,the expandable membrane 18 is typically configured to circumferentiallyextend about the longitudinal axis of at least a portion of theintermediate body 14. In this embodiment, the expandable membrane 18 maybe configured as a tube that extends from the proximal body 12 to thedistal body 16 with the intermediate body 14 extending through the lumenof the tube. In this configuration, the inner surface of the tube willdefine the expansion chamber 20 as prosthetic nucleus material 60 isinfused into the mobility preservation apparatus 10.

In embodiments without an intermediate body 14 of elongated body 12, theexpandable membrane 18 is typically configured to function as theintermediate body 14 by connecting proximal body 12 to distal body 16.In this embodiment, the expandable membrane 18 may be configured as atube extending between the proximal body 12 and distal body 16 to whichit is secured. Again, in this configuration, the inner surface of thetube will define the expansion chamber 20 as prosthetic nucleus material60 is infused into the mobility preservation apparatus 10. Inembodiments with only a single proximal body 12, the expandable membrane18 may be secured to an end of the proximal body 12. The expandablemembrane 18 may expand outward from the end of the proximal body 12 tocontact intervertebral disc space 860 left by a nucleectomy procedure.In this embodiment, the expandable membrane 18 is similar to a balloonwith the inner surface of the balloon defining the expansion chamber 20.

The expandable membrane 18 is typically formed from an elastomericmaterial having desired expandability, flexibility and durabilitycharacteristics. The expandable membrane 18 may be formed from anelastomeric material that is deployable, stable and biocompatible. Theelastomeric material may have a durometer shore A hardness in the rangeof substantially about 20-90. In certain configurations as will berecognized by those skilled in the art, this range of durometer shore Ahardness will result in a compliant membrane. One example of a suitableelastomeric material is silicone rubber exhibiting elongation of betweenabout 500% and about 1500%, and most preferably at about 1000%, andhaving a wall thickness of 0.220″. The specific configuration andphysical properties of the expandable membrane may vary depending on theintended treatment or function. For example, the expandable membrane 18may be configured from a moisture-permeable bioabsorbable material whichcould remain in place to assume physiologic loads while the prostheticnucleus material 60 is swelling and the access site is healing. Theexpandable membrane 18 may also be coated with a therapeutic agent to bedelivered, administered to the structures in which it is in conformalcontact when fully filled and deployed. In one exemplary embodiment, thetherapeutic agent could be coated on the external surface of theexpandable membrane.

In another embodiment, the mobility preservation apparatus 10 isconfigured as a pre-formed expandable membrane 18 component containingpre-inserted prosthetic nucleus material 60 sealed within an expansionchamber 20. More specifically, the expandable membrane 18 may beconfigured as an envelope comprising tightly woven or knit fabric ofpolymeric fibers, such as for example Dacron™ or other material with acomparable tensile modulus. The expandable membrane 18 configured as anenvelope may be folded about the intermediate body 14 as illustrated inFIG. 21 for implantation purposes. The envelope may be formed from othertightly woven, high molecular weight, high tenacity, flexible polymericfabric, such as for example high molecular weight polyethylene,polyester, polyolefin, polyethylene terephthalate,polytetrafluoroethylene, polysulfone, nylons, or any other highmolecular weight, and other high tenacity materials including carbonfiber yarns, ceramic fibers, metallic fibers, etc. The fabric may havepores that are small enough to confine the prosthetic nucleus materialwithin the envelope while allowing passage (unidirectional orbi-directional) of low molecular weight hydration fluids or therapeuticagents. In one exemplary embodiment, the openings have an averagediameter of approximately 10 micrometers, although other dimensions areacceptable. While the fabric is described as woven, any otherconfiguration having a semi-permeable or microporous attribute can beused. The selected material may allow for expansion and contraction ofthe prosthetic nucleus material 60 in a controlled fashion. Theprosthetic nucleus material 60 may more effectively cushion the disc inthe manner of the native nucleus pulposus when the envelope fabric isflexible and semi-compliant. Typically, the fabric will have a burststrength that is greater than the swelling pressure of the prostheticnucleus material 60 when fully hydrated to prevent rending and loss ofthe hydrogel core. By having an envelope or jacket that is both flexibleand semi-compliant, and, in some instances, with the elasticity beingmodulus matched to the native nucleus, the filled envelope can beeffective as a dampener, and may properly assume and distribute loads.When fully hydrated, an inelastic envelope expanded to its full capacitymay not stretch or give when a load is applied and cannot deform, sincethe compression modulus in the normal load range is nearly vertical orincompressible, or distribute loads uniformly, thereby likely resultingin device subsidence and/or transition syndrome.

In yet another embodiment, the expandable membrane 18 may be composed ofnon-woven fibers. These non-woven fibers may be of a similar type tothose used in the manufacture of Tyvek™ film. In another embodiment, amoisture-permeable polymeric expandable membrane 18 may comprise abiaxially oriented membrane modified to be microporous by mechanicalmeans, such as of laser drilling, or by chemical methods. In one aspect,the chemical methods may involve the leaching out of sacrificial saltparticles to achieve a satisfactory end configuration. These chemicalmethods may result in openings in the 10 micrometer range. Under certainconditions, such end configurations may promote tissue ingrowth.

In another embodiment, the expandable membrane 18 is composed of amoisture permeable material that is partially filled with a prostheticnucleus material 60 prior to axial implantation and deployment into theprepared space. The expandable membrane 18 may also be sealed prior toimplantation. The prosthetic nucleus material 60 of this variation willtypically be hydroscopic. Due to an infusion of moisture fromphysiologic fluid present in the disc space 860 through the moisturepermeable material of the expandable membrane 18, the prosthetic nucleusmaterial 60 swells in situ, to expand a semi-compliant expandablemembrane 18. The expanded expandable membrane 18 may then providedistraction and maximum surface area. In some aspects, the expandablemembrane 18 may expand to conformably contact the surfaces andstructures of the intervertebral disc space 860. Accordingly, theexpandable membrane 18 may effectively distribute physiologic loads.

In embodiments with at least a proximal body 12 and a distal body 16,the expandable membrane 18 is typically sealing secured one or more ofthe proximal body 12, distal body 16 and the intermediate body 14. Inone aspect, the expandable membrane 18 is sealing secured at itsproximal end to the distal end of proximal body 12 and at its distal endto the proximal end of distal body 16. In another aspect, the expandablemembrane 18 is sealingly secured at its proximal end about the proximalend of the intermediate body 14 and at its distal end about the distalend of the intermediate body 14. When fenestrations 74 are included onthe intermediate body 14, expandable membrane 18 is typically secured atlocations both proximal and distal to any fenestrations 74 or otherpassages through which prosthetic nucleus material 60 may pass to expandof the expandable membrane 18. In embodiments with only a proximal body12, expandable membrane 18 is typically attached to the distal end ofthe proximal body 12 and is in fluid communication with proximal lumen42 for purposes of expansion.

The expandable membrane 18 may be mechanically, chemically or otherwisesecured to one or more of the proximal body 12, distal body 16 and theintermediate body 14. In one aspect, retainer rings may be providedabout the expandable membrane 18 to secure the expandable membrane 18 tothe adjacent components as illustrated in FIGS. 5 to 10, 12, 13, 19 and20. FIGS. 5 to 13 illustrate some details of embodiments for securingthe expandable membrane 18 to or into the adjacent structure. Typically,a first retainer ring 52 will secure the proximal end of the expandablemembrane 18 and a second retainer ring 56 will secure the distal end ofthe expandable membrane 18. In one aspect, the retainer rings may belaser-welded to further secure the ring about the expandable membrane18. As illustrated in FIGS. 5 to 10 for exemplary purposes, theexpandable membrane 18 may be adhesively affixed and/or engaged by laserwelding of a retainer ring, to or into the proximal body 12 and thedistal body 16. The expandable membrane 18 may be secured with anadhesive. One suitable adhesive is a silicone adhesive.

The expandable membrane 18 may be expanded by injection of a prostheticnucleus material 60. In one aspect, the injection of prosthetic nucleusmaterial 60 may be under high pressure. Typically, the prostheticnucleus material 60 is injected into the expandable membrane 18 with asyringe. In this manner, the expandable membrane 18 may inflate andextend into the intervertebral disc space 860 until it conformablycontacts the surfaces within the intervertebral disc space 860. Theultimate expansion of the expandable membrane 18 can be limited bycontact with and movement of the end plates of the adjacent vertebralbodies and the annulus. Prosthetic nucleus materials 60 may includesaline, viscoelastic gels, elastomeric solids, or blends or combinationsthereof.

In a preferred aspect, prosthetic nucleus material 60 can be a hydrogel,which can be introduced into the expansion chamber 20 of the expandablemembrane 18 in a liquid or dry particulate form or in microspheres orbeads. Further, regardless of the material, the prosthetic nucleusmaterial preferably has a Shore D range from between A10 to D90. Forexample, one hydrogel is formulated as a mixture of hydrogelpolyacrylonitrile or any hydrophilic acrylate derivative with a uniquemultiblock copolymer structure or any other hydrogel material having theability to imbibe and expel fluids while maintaining its structure undervarious stresses. As an example, the hydrogel can be formulated as amixture of polyvinyl alcohol and water. The hydrogel core formed withinthe expandable membrane 18 will swell as it absorbs fluids through theporous fabric wall of the expandable membrane 18, preferably in themanner of a native nucleus. When fully hydrated, the hydrogel core mayhave a water content of between 25-95%. The hydrogel material of onesuitable embodiment is manufactured under the trade name Hypan® byHymedix International, Inc., and has a water content of about 80-95%.Another hydrogel system comprises natural hyaluronan gels or blends thatmay be chemically altered to enhance structure, e.g., scaffoldingability or physical state, and optimize biomechanical properties in situvia laser exposure, e.g., to convert liquid into a solid. Yet otherhydrogels (such as, PEG; PEO; PVP) or blends of hydrogels (such as,PVA/PVP; PEG-based/PE glycated hydrogels; crosslinked aliphaticpolyoxaamide polymers; or combinations of synthetic and nativepolypeptides or GAGs, such as actin, fibrinogen, collagen and elastin;chondroitin, keratin and dermatan sulfate; chitosan) and/or elastomersor other combinations (e.g., incorporation of an ionic or hydrophobicmonomer into the hydrogel network, to engineer a reversibly responsivepolymer) that optimize desired intramolecular and intermolecular bondingarrangements and reproduce the viscoelastic properties of the nativenucleus, may also be used. Conceptually, this is enabled fromunderstanding fundamental relationships between the structure of thepolymer (e.g., molecular weight; cross-linking density, etc.) underphysiological conditions and the physical properties of the resultinghydrogels. As noted previously, the native nucleus consists of mostlytype II collagen (cartilage like) and large protein macromoleculescalled proteoglycans that absorb water into the disc and are extremelyimportant to the biomechanical properties of the disc. Hydrogels modifytheir molecular arrangement, volume and/or phase when acted upon by aspecific stimulus such as temperature, light, a pH change or otherchemical inducement, osmotic pressure or mechanical stress, or electricfield, and selection of the prosthetic nucleus material of the presentinvention is not limited in scope with respect to materials' triggerstimuli, and may include those that are either chemically and/orphysically cross-linked gels (e.g., via ion-complexation or those thatare thermoreversibly cross-linked) as suitable. Hence in a preferredembodiment, the prosthetic nucleus material is selected based on itsstability under physiological conditions and/or in physiological fluids,including the ability withstand load, resist shear stresses and fatigueforces, or other factors that might otherwise induce fragmentation orotherwise promote extrusion or migration, or fractional mass loss overtime.

In other aspects, the expandable membrane 18 may be configured tocontain prosthetic nucleus material 60 comprising hydrogels that areimplanted or inserted in a dehydrated condition, for example by means ofa glycerin carrier. The hydrogels are typically 3-dimensional structuresconsisting mainly of hydrophilic (i.e., very high affinity for water)polymeric materials or copolymers which retain water, withoutdissolving, within a network stability that is achieved through thepresence of chemical or physical crosslinks (e.g., entanglements;crystallites; primary covalent or secondary hydrogen, ionic, or Van derWaals bonds). In this manner, the overall bulk of the mobilitypreservation apparatus 10 may be reduced, allowing the hydrogel to beinserted through a smaller access and the subsequent hydration, whichresults in an increase in volume of the hydrogel, may assist in discdistraction to at least partially restore disc height. In certainembodiments, the prosthetic nucleus material 60 may remain fluid. Inother embodiments, mobility preservation apparatus 10 may be configuredfor deployment via minimal access, by introducing the hydrogel orpolymeric material in a first state or condition (e.g., flowable), andto then allow or induce conversion of the material to a second phase orstate, (e.g., to a solid). In this manner, the prosthetic nucleusmaterial 60 can be introduced through the smallest possible access andyet still be provided in sufficient quantity to fill the disc space 860and provide the desired function. Examples of methods to convert amaterial from a first flowable state to a second solid state include butare not limited to: a temperature phase change as from a melted state toa frozen state, polymerization of a monomer or low molecular weightpolymer such as with the use of a catalyst; laser or UV cross-linking ofa liquid polymer resulting in a solid, leaching of a solvent byreplacing it with water (for example: polyacrylonitrile-polyacrylamidehydrogel can be dissolved in dimethylsufloxide (DMSO) resulting in aflowable liquid which will instantly transform to a solid in thepresence of water, into which the DMSO will preferentially flow), use ofreverse gelation polymers, such as Pluronic™, commercially availablefrom BASF, Inc., Mount Olive, N.J. (USA), that are liquid at roomtemperature and form a solid at elevated temperatures such as bodytemperature, etc.

As noted earlier, mobility preservation apparatus 10 are typicallyimplanted following complete or partial nucleectomy to remove all or aportion of the nucleus pulposus, respectively, to create a disc space860 within the intervertebral disc. However, generally the bore 850through which a prosthetic nucleus apparatus 10 is axially implantedwill typically be smaller than the volume of the intervertebral discspace 860 to be augmented or replaced. To compensate for the spatialdiscrepancy, in one variation, an expandable membrane 18 is inserted ina folded, relaxed, or unfilled state into the prepared disc space 860and prosthetic nucleus material 60 is injected or infused into themembrane in situ. In situ, the prosthetic nucleus materials 60 aretypically delivered through an elongated delivery tube or catheterconnected to the syringe. An elongated delivery tube may seat againstthe proximal end of the proximal body 12 to transfer the prostheticnucleus material into the proximal lumen 42 of the proximal body 12. Inone aspect, the two components may be seated by rotatably engaging oneanother such as for example by including a Leur Lock type configuration.Alternatively, the elongated delivery tube may be configured to bereceived within a lumen (e.g. proximal lumen 42, intermediate lumen 44and/or distal lumen 46) as illustrated in FIGS. 22 to 26. Depending uponthe particular embodiment, the prosthetic nucleus material 60 may becommunicated into the expansion chamber 20 defined by the expandablemembrane 18 directly from the proximal lumen 42, through fenestrations74 in communication with proximal lumen 42 and/or intermediate lumen 44,or in other ways communicated to the expandable membrane 18 as will berecognized by those skilled in the art upon review of the presentdisclosure. The distal end of the elongated delivery tube may beinserted through a cannula into the proximal body 12 as illustrated inFIGS. 22 to 26. As the prosthetic nucleus material 60 is providedthrough the proximal lumen 42 or fenestrations 74 the material engagesthe inner surface of expandable membrane 18 and, with sufficientinternal pressure, causes the expandable membrane 18 to expand byfilling the expansion chamber 20.

FIGS. 1 to 4 illustrate an exemplary mobility preservation apparatus 10.The mobility preservation apparatus 10 includes the proximal body 12,intermediate body 14, and distal body 16. The mobility preservationapparatus 10 will typically have an aspect ratio of greater than 1. Themobility preservation apparatus 10 may be formed from a single piece ofmaterial, as illustrated in FIGS. 1 and 2, or may be formed by multiplecomponents coupled together as illustrated in FIG. 3. When formed asmultiple components, the proximal component and the distal component maybe secured to one another to form the mobility preservation apparatus10.

In the multiple component configurations, one component may include amale portion 24 while the corresponding component will include femaleportion 26. The male portion 24 is then received in the female portion26 to couple the two components together. As illustrated in FIG. 3, theintermediate body 14 and the distal body 16 are machined from a singlepiece of material and include a male portion 24. The proximal body 14 isitself illustrated as machined from a single piece of material andincludes a female portion 26. The female portion 26 is generallyconfigured to receive in the male portion 24. In one aspect, the femaleportion 26 may be sized and/or shaped to rotatably receive the maleportion 24. For example, both the male portion 24 and the female portion26 could have a circular shape about their longitudinal axis to permitrotating when coupled or the female portion 26 could have a circularshape about its longitudinal axis and the male portion 24 could have ahexagonal shape about its longitudinal axis but could be sized to bereceived within the female portion 26. In this manner, the proximalcomponent and the distal component may be rotated independently of oneanother which has advantages for spacing and distraction purposes aswill be recognized by those skilled in the art. In another aspect, thefemale portion 26 may be sized and/or shaped to interlockingly receivethe male portion 24. For example, both the male portion 24 and thefemale portion 26 could have a hexagonal shape about their longitudinalaxis with corresponding sizes to permit the male portion 24 to beinterlocking received within the female portion 26 when the twocomponents are coupled. In this manner, the proximal component and thedistal component may be synchronously rotated and, yet, could remainindependently implantable.

In operation, embodiments of the multi-component configurations ofelongated body 12 may, for example, allow more effective use of the S1vertebra to drive and lift the L5 segment in a controlled manner andindependent of patient anatomy or the location of first threadengagement on/into the L5 vertebral body. Further, the modularity of themulti-component configurations of elongated body 12 may facilitateretention of progression-in-treatment options. For example, a pluginserted into the proximal end of a cannulated female inferior anchorportion, which plug extends sufficiently through and distally into thefenestrated intermediate body 14 of the mobility preservation apparatusto effectively reduce the flexibility of the device. In the extreme,such a plug could effectively convert the spinal mobility apparatus 10into an immobilization or fusion device.

The proximal body 12 of FIGS. 1 to 3 includes a set of first threads 32to secure proximal body 12 to the body of a caudal vertebra 800. Thefirst threads 32 are illustrated for exemplary purposes as extendingover substantially the entire length of proximal body 12. The firstthreads 32 may include a chamfer to ease insertion and removal of themobility preservation apparatus 10 from various tools involved with itsimplantation. The proximal body 12 may define a proximal lumen 42, asillustrated in FIGS. 1 and 2. The proximal lumen 42 of the proximal body12 may be configured to receive a driver 400, such as for example ahex-head driver, to confer a rotational force to insert, remove orposition a mobility preservation apparatus 10 within or betweenvertebral bodies. Alternatively, proximal lumen 42 may be configured toallow the passage of a driver 400 to be received by the intermediatelumen 44 of intermediate body 14 and/or the distal lumen 46 of distalbody 16. In addition, proximal lumen 42 of the proximal body 12 mayinclude internal threads 50 to secure a plug 80 and/or an elongatedmember 82, as illustrated in FIGS. 11, 15 and 16 and discussed below.

The intermediate body 14 of FIGS. 1, 2, and 4 is secured to proximalbody 12. The intermediate body 14 is illustrated as a helical flexurefor exemplary purposes. The helical flexure is illustrated as extendingover substantially the entire length of intermediate body 14. However,those skilled in the art will recognize that the helical flexure orother flexible element 22 of intermediate body 14 may extend over theentire length or a fraction of the length of intermediate body 14. Theintermediate body 14 may also define an intermediate lumen 44. Theintermediate lumen 44 may be coaxial and in communication with theproximal lumen 42 of proximal body 12. Intermediate lumen 44, asillustrated in FIGS. 2 and 4, may be configured to receive a driver 400,such as for example a hex-head driver, to confer a rotational force toinsert, remove or position a mobility preservation apparatus 10 withinor between vertebral bodies. Alternatively, intermediate lumen 44 may beconfigured to allow the passage of a driver 400 to be received by thedistal body 16. In addition, intermediate lumen 44 may include internalthreads 50 to secure a plug 80 and/or an elongated member 82, asillustrated in FIGS. 11, 15 and 16 and discussed below. One or morefenestrations 74 may extend between an outer surface of intermediatebody 14 and the intermediate lumen 44. Fenestrations 74 in FIGS. 1 to 3are illustrated as the space defined between the windings of helicalflexure for exemplary purposes. Upon review of the present disclosure,those skilled in the art will recognize that fenestrations 74 may take awide variety of forms.

The distal body 16 of FIGS. 1 to 4 includes a set of second threads 36to secure distal body 16 to the body of a cephalad vertebra 900. Forexemplary purposes, the second threads 36 are illustrated as extendingover substantially the entire length of distal body 16. In oneembodiment, the second threads 36 may have an outside diameter that isless than or equal to the inside diameter of the proximal threads. Thus,the second threads 36 may be sized to slidably pass through a borethrough which the caudad thread must be rotatably driven. Further, thesecond threads 36 may include a chamfer to ease insertion and removal ofthe mobility preservation apparatus 10 from various tools involved withits implantation as well as to assist in placement of the mobilitypreservation apparatus within a patient. The pitch of the second threads36 may be less than the pitch of the first threads 32 to provide adegree of distraction as the mobility preservation apparatus 10 issecured into adjacent vertebrae. In one embodiment, the distal body 16may include second threads 36 having a 12-pitch and the proximal body 12may include first threads 32 having a 10-pitch. Further, the distal body16 may be configured as a self-tapping bone screw by, for example,including a chip breaker at the distal end of the distal body 16. Asillustrated in FIGS. 2 and 4, the distal body 16 may also define adistal lumen 46. The distal lumen 46 of the distal body 16 may beconfigured to receive a driver 400 to confer a rotational force toinsert, remove or position a mobility preservation apparatus 10 withinor between vertebral bodies. In addition, distal lumen 46 may includeinternal threads 50 to secure a plug 80, as illustrated in FIGS. 11, 15and 16 and discussed below. Alternatively, the distal body 16 may definea distal cavity 48, having a closed end as illustrated in FIG. 3. Thedistal cavity 48 may be configured to receive a driver 400 to confer arotational force to insert, remove or position a mobility preservationapparatus 10, as a whole, or the distal body 16 independently within orbetween vertebral bodies. Distal cavity 48 may also prevent themigration of infused materials substantially beyond the intermediatebody 14, such as in procedures designed to add a prosthetic nucleus ontomobility preservation apparatus 10. In addition, distal cavity 48 of thedistal body 16 may include internal threads 50 to secure a plug 80and/or a stiffening member 82, as illustrated in FIGS. 11, 15 and 16 anddiscussed below.

FIGS. 5 to 8 illustrate another exemplary mobility preservationapparatus 10. As illustrated, mobility preservation apparatus 10includes a proximal body 12, an intermediate body 14, a distal body 16and an expandable membrane 18. The mobility preservation apparatus 10will typically have an aspect ratio of greater than 1. The expandablemembrane 18 is secured about the intermediate body 14. The proximal body12, intermediate body 14, distal body 16 may be formed from a singlepiece of material, as illustrated in FIGS. 5 to 7, or may be formed bymultiple components coupled together, as illustrated in FIG. 8. Asillustrated, the proximal component and the distal component may includea male portion 24 and the corresponding component will include femaleportion 26 to couple the proximal component and the distal component.

In a multiple component embodiment, the female portion 26 is generallyconfigured to receive in the male portion 24. In one aspect, the femaleportion 26 may be sized and/or shaped to rotatably receive the maleportion 24. For example, both the male portion 24 and the female portion26 could have a circular shape about their longitudinal axis to permitrotating when coupled or the female portion could have a circular shapeabout its longitudinal axis and the male portion 24 could have ahexagonal shape about its longitudinal axis but could be sized to bereceived within the female portion 26. In this manner, the proximalcomponent and the distal component may be rotated independently of oneanother which has advantages for spacing and distraction purposes aswill be recognized by those skilled in the art. In another aspect, thefemale portion 26 may be sized and/or shaped to interlockingly receivethe male portion 24. For example, both the male portion 24 and thefemale portion 26 could have a hexagonal shape about their longitudinalaxis with corresponding sizes to permit the male portion 24 to beinterlocking received within the female portion 26 when the twocomponents are coupled. In this manner, the proximal component and thedistal component may be synchronously rotated and, yet, could remainindependently implantable.

In operation, embodiments of the multi-component configurations ofmobility preservation apparatus 10 may, for example, allow moreeffective use of the S1 vertebra to drive and lift the L5 segment in acontrolled manner and independent of patient anatomy or the location offirst thread engagement on/into the L5 vertebral body. Further, themodularity of the multi-component configurations of elongated body 12may facilitate retention of progression-in-treatment options. Forexample, a plug 80 may be inserted into proximal lumen 42 at theproximal end of the proximal body 12 and may extend distally asufficiently distance through the intermediate lumen 44 of theintermediate section 14 to compromise the flexibility of flexibleelement 22. If the plug 80 sufficiently compromises the flexibility ofthe flexible element, mobility preservation apparatus 10 may effectivelyserves as an immobilization or fusion device.

The proximal body 12 of FIGS. 5 to 8 includes a set of first threads 32to secure proximal body 12 to the body of a caudal vertebra 800. Thefirst threads 32 are illustrated for exemplary purposes as extendingover substantially the entire length of proximal body 12. The firstthreads 32 may include a chamfer to ease insertion and removal of themobility preservation apparatus 10 from various tools involved with itsimplantation. The proximal body 12 may define a proximal lumen 42. Theproximal lumen 42, as illustrated in FIG. 6 to 8, may be configured toreceive a driver 400, such as for example a hex-head driver, to confer arotational force to insert, remove or position a mobility preservationapparatus 10 within or between vertebral bodies. Alternatively, proximallumen 42 may be configured to allow the passage of a driver 400 to bereceived by the intermediate body 14 and/or distal body 16. In addition,proximal lumen 42 of may include internal threads 50 to secure a plug80, as illustrated in FIGS. 3 and 4 and discussed below.

The intermediate body 14 of FIGS. 5 to 8 is secured to proximal body 12.The intermediate body 14 is illustrated as a helical flexure forexemplary purposes. The helical flexure is illustrated as extending oversubstantially the entire length of intermediate body 14. However, thoseskilled in the art will recognize that the helical flexure or otherflexible element 22 of intermediate body 14 may extend over the entirelength or a fraction of the length of intermediate body 14. Theintermediate body 14 may also define an intermediate lumen 44. Theintermediate lumen 44 may be coaxial and in communication with theproximal lumen 42. The intermediate lumen 44, as illustrated in FIGS. 6to 8, may be configured to receive a driver 400, such as for example ahex-head driver, to confer a rotational force to insert, remove orposition a mobility preservation apparatus 10 within or betweenvertebral bodies. Alternatively, intermediate lumen 44 may be configuredto allow the passage of a driver 400 to be received by the distal body16. In addition, the intermediate lumen 44 may include internal threads50 to secure a plug 80, as illustrated in FIGS. 11, 15 and 16 anddiscussed below. One or more fenestrations 74 may extend between anouter surface of intermediate body 14 and the intermediate lumen 44.Fenestrations 74 in FIGS. 6 to 8 are illustrated as the space definedbetween the windings of helical flexure for exemplary purposes. Uponreview of the present disclosure, those skilled in the art willrecognize that fenestrations 74 may take a wide variety of forms.

The distal body 16 of FIGS. 5 to 8 includes a set of second threads 36to secure distal body 16 to the body of a cephalad vertebra 900. Forexemplary purposes, the second threads 36 are illustrated as extendingover substantially the entire length of distal body 16. In oneembodiment, the second threads 36 may have an outside diameter that isless than or equal to the inside diameter of the proximal threads. Thus,the second threads 36 may be sized to slidably pass through a borethrough which the caudad thread must be rotatably driven. Further, thesecond threads 36 may include a chamfer to ease insertion and removal ofthe mobility preservation apparatus 10 from various tools involved withits implantation as well as to assist in placement of the mobilitypreservation apparatus within a patient. The pitch of the second threads36 may be less than the pitch of the first threads 32 to provide adegree of distraction as the mobility preservation apparatus 10 issecured into adjacent vertebrae. In one embodiment, the distal body 16may include second threads 36 having a 12-pitch and the proximal body 12may include first threads 32 having a 10-pitch. Further, the distal body16 may be configured as a self-tapping bone screw by, for example,including a chip breaker at the distal end of the distal body 16. Asillustrated in FIG. 8, the distal body 16 may also define a distal lumen46. The distal lumen 46 may be configured to receive a driver 400 toconfer a rotational force to insert, remove or position a mobilitypreservation apparatus 10 within or between vertebral bodies. Inaddition, distal lumen 46 may include internal threads 50 to secure aplug 80, as illustrated in FIGS. 11, 15 and 16 and discussed below.Alternatively, the distal body 16 may define a distal cavity 48, havinga closed end as illustrated in FIGS. 6 and 7. The distal cavity 48 maybe configured to receive a driver 400 to confer a rotational force toinsert, remove or position a mobility preservation apparatus 10, as awhole, or the distal body 16 independently within or between vertebralbodies. Distal cavity 48 may also function to prevent the migration ofinfused materials substantially beyond the intermediate body 14, such asin procedures designed to add a prosthetic nucleus onto mobilitypreservation apparatus 10. In addition, distal cavity 48 of the distalbody 16 may include internal threads 50 to secure a plug 80 (not shown).

An embodiment of an expandable membrane 18 is illustrated in FIGS. 5 to8 for use in conjunction with the present embodiment having a proximalbody 12, an intermediate body 14 and a distal body 16. As illustrated,expandable membrane 18 is secured over the length of intermediate body14. As illustrated in FIGS. 5 to 8, the expandable membrane 18 issecured at locations both proximal and distal to fenestrations 74.Accordingly, prosthetic nucleus material 60 passing throughfenestrations 74 may forcibly expand the expandable membrane 18. Forexemplary purposes, the expandable membrane 18 is illustrated as beingsecured about the intermediate body 14 by a proximal retaining ring 22and a distal retaining ring 24. The proximal retaining ring 22 and thedistal retaining ring 24 are secured over the expandable membrane 18. Inone aspect, the proximal retaining ring 52 and the distal retaining ring54 may be laser welded to secure them in position. Alternatively, theproximal retaining ring 52 and the distal retaining ring 54 may be sizedand configured to compressionally secure portions of the expandablemembrane 18 about the intermediate body 14. FIGS. 6 and 8 illustrate theexpandable membrane 18 in an unexpanded position. Further, for exemplarypurposes, the unexpanded position is illustrated with the expandablemembrane positioned flush with the intermediate body 14. FIGS. 5 and 6illustrate the expandable membrane in an at least partially expandedposition.

FIGS. 9 and 10 illustrate yet another embodiment of a mobilitypreservation apparatus 10. The mobility preservation apparatus 10 ofFIGS. 9 and 10 includes a proximal body 12, a distal body 16 and anexpandable membrane 18. FIG. 9 illustrates the expandable membrane 18 inan unexpanded position. FIG. 10 illustrates the expandable membrane 18in an at least partially expanded position. As illustrated, the proximalbody 12 and the distal body 16 are connected by a tubular expandablemembrane 18 to form a mobility preservation apparatus 10. The expandablemembrane 18 is secured at a proximal end to proximal body 12 and at adistal end to distal body 16.

The proximal body 12 of FIGS. 9 and 10 includes a set of first threads32 to secure proximal body 12 to the body of a caudal vertebra 800. Thefirst threads 32 are illustrated for exemplary purposes as extendingover substantially the entire length of proximal body 12. The firstthreads 32 may include a chamfer to ease insertion and removal of themobility preservation apparatus 10 from various tools involved with itsimplantation. The proximal body 12 may define a proximal lumen 42. Theproximal lumen 42, as illustrated in FIGS. 9 and 10, may be configuredto receive a driver 400, such as for example a hex-head driver, toconfer a rotational force to insert, remove or position a mobilitypreservation apparatus 10 within or between vertebral bodies.Alternatively, proximal lumen 42 may be configured to allow the passageof a driver 400 to be received by the intermediate body 14 and/or distalbody 16. In addition, proximal lumen 42 of may include internal threads50 to secure a plug 80, as illustrated in FIGS. 3 and 4 and discussedbelow.

The distal body 16 of FIGS. 9 and 10 includes a set of second threads 36to secure distal body 16 to the body of a cephalad vertebra 900. Forexemplary purposes, the second threads 36 are illustrated as extendingover substantially the entire length of distal body 16.

In one embodiment, the second threads 36 may have an outside diameterthat is less than or equal to the inside diameter of the proximalthreads. Thus, the second threads 36 may be sized to slidably passthrough a bore through which the caudad thread must be rotatably driven.Further, the second threads 36 may include a chamfer to ease insertionand removal of the mobility preservation apparatus 10 from various toolsinvolved with its implantation as well as to assist in placement of themobility preservation apparatus within a patient. The pitch of thesecond threads 36 may be less than the pitch of the first threads 32 toprovide a degree of distraction as the mobility preservation apparatus10 is secured into adjacent vertebrae. In one embodiment, the distalbody 16 may include second threads 36 having a 12-pitch and the proximalbody 12 may include first threads 32 having a 10-pitch. Further, thedistal body 16 may be configured as a self-tapping bone screw by, forexample, including a chip breaker at the distal end of the distal body16. As illustrated in FIG. 8, the distal body 16 may also define adistal lumen 46. The distal lumen 46 may be configured to receive adriver 400 to confer a rotational force to insert, remove or position amobility preservation apparatus 10 within or between vertebral bodies.In addition, distal lumen 46 may include internal threads 50 to secure aplug 80, as illustrated in FIGS. 11, 15 and 16 and discussed below.Alternatively, the distal body 16 may define a distal cavity 48, havinga closed end as illustrated in FIGS. 6 and 7. The distal cavity 48 maybe configured to receive a driver 400 to confer a rotational force toinsert, remove or position a mobility preservation apparatus 10, as awhole, or the distal body 16 independently within or between vertebralbodies. Distal cavity 48 may also function to prevent the migration ofinfused materials substantially beyond the distal body 16, such as inprocedures designed to add a prosthetic nucleus onto mobilitypreservation apparatus 10. In addition, distal cavity 48 of the distalbody 16 may include internal threads 50 to secure a plug 80 (not shown).

As illustrated, expandable membrane 18 is secured between the proximalbody 12 and the distal body 16. The expandable membrane 18 may besecured circumferentially about the adjacent components, within a lumenor cavity associated with the component or may otherwise be secured aswill be recognized by those skilled in the art. The expandable membrane18 may include a proximal lip 62 and a distal lip 66 to engage theproximal body 12 and the distal body 16, respectively. The proximal body12 may include a proximal groove 64 to receive the proximal lip 62 ofthe expandable membrane 18. Typically, the proximal groove 64 willextend circumferentially around the proximal body 12 to sealing engageexpandable membrane 18. Similarly, the distal body 16 may include adistal groove 68 to receive the distal lip 66 of the expandable membrane18. Typically, the distal groove 68 will extend circumferentially aroundthe distal body 16 to sealing engage expandable membrane 18. Theexpandable membrane 18 may be further secured about the intermediatebody 14 by a proximal retaining ring 22 and a distal retaining ring 24.The proximal retaining ring 22 and the distal retaining ring 24 aresecured over the expandable membrane 18. In one aspect, the proximalretaining ring 52 and the distal retaining ring 54 may be laser weldedto secure them in position. Alternatively, the proximal retaining ring52 and the distal retaining ring 54 may be sized and configured tocompressionally secure portions of the expandable membrane 18 about theintermediate body 14. In addition or alternatively, an adhesive may beprovided to bind the expandable membrane 10 to the proximal body 12 andthe distal body 16. Regardless of how the expandable membrane 18 issecured, the expandable membrane 18 is sufficiently secure to permitprosthetic nucleus material 60 passing through proximal lumen 42 mayforcibly expand the expandable membrane 18 without dislodging theexpandable membrane 18.

An embodiment of an expandable membrane 18 is illustrated in FIGS. 9 and10 for use in conjunction with the present embodiment having a proximalbody 12 and a distal body 16. The expandable membrane will typicallylack sufficient torsional rigidity to permit the implantation ofmobility preservation apparatus using a driver 400 which engages onlythe proximal body 12. Accordingly, a driver 400 will typically beprovided to simultaneously engage both the proximal body 12 and thedistal body 16 to synchronously rotate the proximal body 12 and distalbody 16 during implantation. This may be accomplished by having twoheads on driver 400 the more distal head sized to pass through proximallumen 42 and to engage distal lumen 46 and the more proximal head beingsized to engage proximal lumen 42. In one aspect, driver 400 may beconfigured to engage the mobility preservation apparatus 10 to preventthe dislodgement of the heads during implantation.

In one exemplary methodology, the mobility preservation apparatus 10illustrated in FIGS. 1 to 10 may be implanted trans-sacrally. Whereafter paracoccygeal entry, a mobility preservation apparatus 10 may bedeployed into the L5-S1 spinal motion segment via the trans-sacral axialtechniques and instrumentation disclosed in co-pending and commonlyassigned United States patent application entitled “Access Assembly forGuiding Instrumentation Through Soft Tissue to a Point on the Spine”,attorney docket number AXIAMD.015A, filed concurrently herewith on Oct.22, 2004, the contents of which are hereby incorporated in entirety intothis disclosure by reference.

More specifically, in a preferred procedure the blunt-tipped stylet isintroduced percutaneously through the guide pin introducer and safelyenables advancement of the guide pin introducer through the soft tissueof the pre-sacral track up to the anterior target site on sacral face.The stylet is withdrawn from the guide pin introducer now positioned inplace at the target site, and a beveled guide pin of about 0.125″ O.D.is inserted, with its handle attached by engagement means of a hex andthumb screw lock, into the guide pin introducer, forming a guidepin-guide pin introducer assembly. Under fluoroscopic guidance toappropriately maintain anterior/posterior alignment, the beveled tip atthe distal end of the guide pin is advanced into the sacrum from thetarget site on the sacral face by means of tapping the proximal end witha hammer or mallet. The guide pin is driven through the sacrum; theL5-S1 disc space, and anchored into L5 prior to bone dilation. The thumbscrew is loosened and the guide pin handle is removed from the guidepin's proximal end, to enable attachment of the guide pin extension. Theguide pin introducer is then removed without disturbing the guide pin.

Next, a cannulated 6 mm dilator is inserted over the guide pin extensionand its distal end is advanced, by means of tapping with a slap hammer,also inserted over the guide wire, against the handle at the proximalend of the dilator, up to (or in close proximity, e.g., within about6-10 mm) the vertebral end plate. The 6 mm dilator is then withdrawnover the guide pin, and the same process is sequentially repeated usingfirst a cannulated 8 mm dilator, and then a cannulated 10 mm dilator aspart of an assembly in engagement with a tapered (approximately 0.375″OD) dilator sheath, respectively inserted over the guide pin andadvanced into the sacrum. The dilator handle is then disengaged from thesheath and the dilator is withdrawn over the guide pin, leaving thedilator sheath in place, to preserve trajectory. The guide pin is thenremoved to enable insertion of a (non-cannulated) 9 mm twist drillthrough the dilator sheath, which is used to drill, through the largedilator sheath, through S1 into the L5-S1 intervertebral disc, throughthe inferior endplate of L5 and to axially advance into L5. For L5-S1procedures, i.e., where multi-level implants or therapies are notprescribed, the axial access track is not drilled through L5 into theL4-L5 intervertebral disc, and stops often about 10 mm caudally from thesuperior vertebral endplate of L5, and the length of the axial trackaccommodates the distal anchor of the particular mobility preservationapparatus 10 to be implanted.

Then, following removal of the 9 mm drill, the 10 mm dilator taperedlarge dilator sheath is further advanced, if necessary, through whichnucleectomy is performed, by using 3-4 cutters with a plurality of(preferably teardrop shaped) blade configurations, e.g., lengths, andabout 6 tissue extraction tools to create an intervertebral disc space860. After removal of the nucleus pulposus (typically taking care not toscrape cartilage from the vertebral endplates so as to cause bleedingthat would enhance undesired fusion) and withdrawal of tissue cutter andextraction tools, typically, an approximately 23″ long guide pin ofabout 0.09″ diameter is inserted through the dilator sheath, and theexchange bushing is inserted over the pin, over the dilator sheath andadvanced to the target site on the sacral face. Then, the exchangecannula is inserted over the exchange bushing, and left in placefollowing subsequent removal of the bushing. The 10 mm dilator is theninserted into the dilator sheath, to engage and remove it.

At this point in the procedure as generally illustrated in FIGS. 22 to26, mobility preservation apparatus 10 are delivered generally viatorque driver inserter tools such as driver 400 appropriately configuredwith male components to engage (e.g., via hex head) female sockets ofthe mobility preservation apparatus 10 (or a female component on adriver 400 engaging a male component on a mobility preservationapparatus 10) within a cannula, over the guide wire and deployed throughthe exchange system and into or through the intervertebral disc space860. The guide pin is removed and disc spaces are augmented withprosthetic nucleus material 60, e.g., by injection through a suitableinsertion media injector either into/within an expandable membrane 18 ofan accompanying mobility preservation apparatus 10, or via deploymentdirectly into the intervertebral disc space 860, e.g., by engaging thedistal end of the inserter with the proximal end of the mobilitypreservation apparatus 10 and injecting the augmentation media therein,e.g., delivery into a membrane 18, or through fenestrations 74, ordirectly into the intervertebral disc space 860. Deployment ofprosthetic nucleus material 60 directly into the intervertebral discspace 860 may be performed by various means and techniques, e.g., anexemplary multi-step procedure wherein a barrier sealant membrane isintroduced to seal (e.g., annular) fissures followed by subsequentintroduction of bulk prosthetic nucleus material 60, such as disclosedin co-pending and commonly assigned U.S. Provisional Patent Applicationhaving Ser. No. 60/599,989 filed Aug. 9, 2004 the contents of which arehereby incorporated in entirety into this disclosure by reference.

For example, in deploying a mobility preservation apparatus 10, thedistal end of the mobility preservation apparatus 10 is deployed overthe proximal end of the extended guide pin and delivered into andthrough the exchange cannula. More specifically, the proximal end of theproximal body 12 is engaged to a torque (e.g., rotatable) rod drivertool, and the preloaded mobility preservation apparatus 10 is insertedover the guide pin to axially advance, by means of rotation, the implantthrough S1, through the L5-S1 disc intervertebral disc space 860 andinto L5. When mobility preservation apparatus 10 are configured tocomprise a proximal body 12 having first threads 32 and a distal body 16having second threads 36 where the respective threads may be configuredwith differential pitches, such rotation distracts the L5 and S1vertebral bodies, as the device is driven into L5.

After mobility preservation apparatus 10 deployment in vivo andinsertion of the prosthetic nucleus material 60 by means of anappropriate augmentation media inserter, the extended guide pin isreinserted and stop-flow means 80, e.g., typically a cannulated,female-threaded axial rod plug 80, is next inserted, as follows: theproximal end of the plug 80 is fitted on the distal end of the roddriver that is configured (e.g., male hex head) to engage the plug 80,and which is then inserted and axially advanced into the proximal end ofthe proximal body of the deployed mobility preservation apparatus 10.Next, the plug driver is removed. Once the plug driver has been removedfrom the access track, site closure is performed by the surgeon.

In one aspect of the invention, when a threaded mobility preservationapparatus 10 having a proximal body 12 and a distal body 16 with nointermediate body 14 and which may include an expandable membrane 18,are deployed, the mobility preservation apparatus 10 implants aredeployed by means of a driver 400 that is configured as a dual-headedrod driver, e.g., with rotatable hex heads 404, 406, such as disclosedin co-pending and commonly assigned United States Patent ProvisionalApplication entitled “Multi-Part Assembly for Introducing Axial Implantsinto the Spine”, attorney docket number TranS1041022LLG-6, filedconcurrently herewith on Oct. 22, 2004, the disclosure of which ishereby incorporated by reference. More specifically, the implantdeployment tool comprises a driver shaft configured to securely engagenon-contiguous distal and proximal bodies, respectively, duringdeployment while maintaining their spatial relationship, i.e., theseparation distance, axially, remains constant between the distal andproximal bodies relative one to the other during introduction in vivo.In this manner, mobility preservation apparatus 10 configured withdistal and proximal bodies with differential thread pitches, and whereinthe major diameter of the first threads 32 of the distal body 16 is lessthan the major diameter of the second threads 36 of the proximal body 12and the proximal end of the distal body 16 and the distal end of theproximal body 12 are not mutually engaged, may be axially introduced anddistract the cephalad (e.g., L5) vertebral body relative to the caudal(e.g., S1) vertebral body. In an exemplary aspect, the driver tool isconfigured wherein the more proximal male hex head engaging the proximalbody 12 is of larger diameter than that of the distal male hex headengaging the distal body. The distal body 16 is additionally retained inplace on the distal head of the dual headed driver during axialadvancement (to preclude axial translation) by means of an externallythreaded retainer wire which engages internal threads 50 in a distallumen 46 or distal cavity 48 of the distal body 16. The proximal body 12is retained in place on the proximal head of the dual headed driver bymeans of a retaining coupler with male threads on its distal end thatengage female threads (socket) in the proximal end of the proximal body12. The mobility preservation apparatus 10 is advanced in a preloadedconfiguration on and by means of the driver tool to rotatably engage anddistract the vertebral bodies, after which prosthetic nucleus material60 is sequentially introduced by means as described above, to first fillthe disk space surrounding the implant and driver, then with additionalprosthetic nucleus material 60 deployed to fill space vacated by thedriver 400 once it is disengaged from the proximal and distal bodies andremoved from the axial access tract. The distal body 16 may optionallybe plugged (e.g., with silicone elastomer tamped into the cannulatedspace vacated by the distal retention wire) prior to introduction ofprosthetic nucleus material 60, and after deployment of prostheticnucleus material 60, the proximal end of the proximal body 12 is pluggedand the site closure is performed as noted above.

Still another embodiment of a mobility preservation apparatus 10 isillustrated in FIGS. 11 to 13, and 29. The mobility preservationapparatus of FIGS. 11 to 13 and 29 generally includes a proximal body 12and an expandable membrane 18. In one aspect of the present invention,the mobility preservation apparatus may not include an expandablemembrane 18. In an exemplary configuration, the overall length ofmobility preservation apparatus 10 may range from about 20 mm to about50 mm with the expandable membrane 18 in the expanded position. Themobility preservation apparatus in accordance with the present inventionwill typically have an aspect ratio of greater than 1.

The proximal body 12 is generally configured to be secured within avertebral body to position the expandable membrane 18 in or adjacent tothe disc space, for example the disc space 860 between L5 and S1. Theexpandable membrane 18 is generally configured to expand into the discspace 860 to provide a degree of support to adjacent vertebral bodies.Proximal body 12 is generally configured to secure the proximal end ofmobility preservation apparatus 10 to a caudally positioned vertebra800. As illustrated for exemplary purposes throughout the figures,proximal body 12 may include first threads 30 to secure proximal body 12in a bore 850 through the body of a vertebra.

As illustrated, the proximal body 12 is formed from a single piece ofmaterial. The expandable membrane 18 is secured within the lumen 40 forexemplary purposes. Typically, the proximal body 12 is formed bymachining or molding as a single piece of material. As illustrated, theproximal body 12 includes a set of first threads 32 to secure proximalbody 12 to the body of a caudal vertebra 800. The first threads 32 areillustrated for exemplary purposes as extending over substantially theentire length of proximal body 12. The first threads 32 may include achamfer to ease insertion and removal of the mobility preservationapparatus 10 from various tools involved with its implantation. Theproximal body 12 may define a proximal lumen 42. The proximal lumen 42of the proximal body 12, as illustrated in FIGS. 1 and 2, may beconfigured to receive a driver to confer a rotational force to insert,remove or position a mobility preservation apparatus 10 within orbetween vertebral bodies. In addition, proximal lumen 42 may includeinternal threads 50 to secure a plug 80, as illustrated in FIGS. 11, 15and 16 and discussed below.

The expandable membrane 18 is secured to the distal end of the proximalbody 12. The expandable membrane 18 is typically secured to the proximalbody 12 to permit the expandable membrane 18 to be expanded in situafter the axial implantation of the proximal body within a vertebralbody 800, 900. In one aspect, a proximal portion of the expandablemembrane 18 is secured to the proximal body and is in fluidcommunication with the proximal lumen 42 to permit the introduction of aprosthetic nucleus material 60 into the expandable membrane 18. Theexpandable membrane 18 may be secured to the proximal body 12 usingadhesives, mechanical couplings, or otherwise as will be recognized bythose skilled in the art. One suitable adhesive for securing theexpandable membrane 18 to proximal body 12 is a silicone adhesive.Alternatively, one or more proximal retainer rings 52 may be providedabout or within the expandable membrane 18 to secure it to the proximalbody 12. The retainer ring 52 may be laser-welded to further secure thering about the expandable membrane 18. As shown in cross-section inFIGS. 12 and 13 for exemplary purposes, the expandable membrane 18 mayinclude a proximal lip 62 that is received internally within theproximal lumen 42 by a proximal groove 64. The proximal lip 62 ofexpandable membrane 18 may be compressionally secured in proximal groove64 by a proximal retaining ring 52 that is compressibly fitted withinproximal lumen 42. Further, an adhesive may also be provided to furthersecure the expandable membrane to the proximal body 12. As illustratedin FIGS. 12 and 13, proximal retaining ring 52 includes internal threads50 to which plug 80 may be secured. As illustrated, expandable membrane18 is secured within proximal lumen 42 of proximal body 12. Accordingly,prosthetic nucleus material 60 passing through proximal lumen 42 mayforcibly expand the expandable membrane 18. Alternatively, theexpandable membrane 18 may be externally secured about the proximal body12 with a proximal retaining ring 52. The proximal retaining ring 52 canbe secured over the expandable membrane 18. In one aspect, the proximalretaining ring 22 may be laser welded to secure them in position.Alternatively and as illustrated, the proximal retaining ring 22 may besized and configured to compressionally secure portions of theexpandable membrane 18 about proximal body 12. As illustrated in FIGS.13 and 14, expandable membrane 18 is secured between the proximal body12 and the distal body 16. The expandable membrane 18 may include aproximal lip 62 and a distal lip 66 to engage the proximal body 12 andthe distal body 16, respectively. The proximal body 12 may include aproximal groove 64 to receive the proximal lip 62 of the expandablemembrane 18. Typically, the proximal groove 64 will extendcircumferentially around the proximal body 12 to sealing engageexpandable membrane 18.

Similarly with embodiments having a distal body 16, the distal body 16may include a distal groove 68 to receive the distal lip 66 of theexpandable membrane 18. Typically, the distal groove 68 will extendcircumferentially around the distal body 16 to sealing engage expandablemembrane 18. The expandable membrane 18 may be further secured about theintermediate body 14 by a proximal retaining ring 52 and a distalretaining ring 56. The proximal retaining ring 52 and the distalretaining ring 56 are secured over the expandable membrane 18. In oneaspect, the proximal retaining ring 52 and the distal retaining ring 54may be laser welded to secure them in position. Alternatively, theproximal retaining ring 52 and the distal retaining ring 54 may be sizedand configured to compressionally secure portions of the expandablemembrane 18 about the intermediate body 14. In addition oralternatively, an adhesive may be provided to bind the expandablemembrane 10 to the proximal body 12 and the distal body 16. Regardlessof how the expandable membrane 18 is secured, the expandable membrane 18is sufficiently secure to permit prosthetic nucleus material 60 passingthrough proximal lumen 42 may forcibly expand the expandable membrane 18without dislodging the expandable membrane 18.

In one exemplary methodology, the mobility preservation apparatus 10illustrated in FIGS. 11 to 13 may be implanted trans-sacrally. Whereutilizing paracoccygeal entry, a mobility preservation apparatus 10 maybe deployed into the L5-S1 spinal motion segment via the trans-sacralaxial techniques and instrumentation disclosed in co-pending andcommonly assigned United States patent application entitled “AccessAssembly for Guiding Instrumentation Through Soft Tissue to a Point onthe Spine”, attorney docket number AXIAMD.015A, filed concurrentlyherewith on Oct. 22, 2004, the contents of which are hereby incorporatedin entirety into this disclosure by reference.

More specifically, in a preferred procedure the blunt-tipped stylet isintroduced percutaneously through the guide pin introducer and safelyenables advancement of the guide pin introducer through the soft tissueof the pre-sacral track up to the anterior target site on sacral face.The stylet is withdrawn from the guide pin introducer now positioned inplace at the target site, and a beveled guide pin of about 0.125″ O.D.is inserted, with its handle attached by engagement means of a hex andthumb screw lock, into the guide pin introducer, forming a guidepin-guide pin introducer assembly. Under fluoroscopic guidance toappropriately maintain anterior/posterior alignment, the beveled tip atthe distal end of the guide pin is advanced into the sacrum from thetarget site on the sacral face by means of tapping the proximal end witha hammer or mallet. The guide pin is driven through the sacrum; theL5-S1 disc space, and anchored into L5 prior to bone dilation. The thumbscrew is loosened and the guide pin handle is removed from the guidepin's proximal end, to enable attachment of the guide pin extension. Theguide pin introducer is then removed without disturbing the guide pin.

Next, a cannulated 6 mm dilator is inserted over the guide pin extensionand its distal end is advanced, by means of tapping with a slap hammer,also inserted over the guide wire, against the handle at the proximalend of the dilator, up to (or in close proximity, e.g., within about6-10 mm) the vertebral end plate. The 6 mm dilator is then withdrawnover the guide pin, and the same process is sequentially repeated usingfirst a cannulated 8 mm dilator, and then a cannulated 10 mm dilator aspart of an assembly in engagement with a tapered (approximately 0.375″OD) dilator sheath, respectively inserted over the guide pin andadvanced into the sacrum. The dilator handle is then disengaged from thesheath and the dilator is withdrawn over the guide pin, leaving thedilator sheath in place, to preserve trajectory. The guide pin is thenremoved to enable insertion of a (non-cannulated) 9 mm twist drillthrough the dilator sheath, which is used to drill, through the largedilator sheath, through S1 into the L5-S1 intervertebral disc.

Then, following removal of the 9 mm drill, the 10 mm dilator taperedlarge dilator sheath is further advanced, if necessary, through whichnucleectomy is performed, by using 3-4 cutters with a plurality of(preferably teardrop shaped) blade configurations, e.g., lengths, andabout 6 tissue extraction tools to create an intervertebral disc space860. After removal of the nucleus pulposus (typically taking care not toscrape cartilage from the vertebral endplates so as to cause bleedingthat would enhance undesired fusion) and withdrawal of tissue cutter andextraction tools, typically, an approximately 23″ long guide pin ofabout 0.09″ diameter is inserted through the dilator sheath, and theexchange bushing is inserted over the pin, over the dilator sheath andadvanced to the target site on the sacral face. Then, the exchangecannula is inserted over the exchange bushing, and left in placefollowing subsequent removal of the bushing. The 10 mm dilator is theninserted into the dilator sheath, to engage and remove it.

As is generally illustrated in FIGS. 22 to 26, mobility preservationapparatus 10 may then be delivered generally via torque driver insertertools such as driver 400, appropriately configured with male componentsto engage (e.g., via hex head) female sockets of the mobilitypreservation apparatus 10 (or a female component on a driver 400engaging a male component on a mobility preservation apparatus 10)within a cannula, over the guide wire and deployed through the exchangesystem and into, to or proximate the intervertebral disc space 860. Theguide pin is removed and disc spaces are augmented with prostheticnucleus material 60, e.g., by injection through a suitable insertionmedia injector either into/within an expandable membrane 18 of anaccompanying mobility preservation apparatus 10, or via deploymentdirectly into the intervertebral disc space 860, e.g., by engaging thedistal end of the inserter with the proximal end of the mobilitypreservation apparatus 10 and injecting the augmentation media therein,e.g., delivery into a membrane 18 or directly into the intervertebraldisc space 860. Deployment of prosthetic nucleus material 60 directlyinto the intervertebral disc space 860 may be performed by various meansand techniques, e.g., an exemplary multi-step procedure wherein abarrier sealant membrane is introduced to seal (e.g., annular) fissuresfollowed by subsequent introduction of bulk prosthetic nucleus material60.

For example, in deploying a mobility preservation apparatus 10, thedistal end of the mobility preservation apparatus 10 is deployed overthe proximal end of the extended guide pin and delivered into andthrough the exchange cannula. More specifically, the proximal end of theproximal body 12 is engaged to a torque (e.g., rotatable) rod drivertool, and the preloaded mobility preservation apparatus 10 is insertedover the guide pin to axially advance, by means of rotation, the implantinto S1.

After mobility preservation apparatus 10 deployment in vivo andinsertion of the prosthetic nucleus material 60 by means of anappropriate augmentation media inserter, the extended guide pin isreinserted and stop-flow means 80, e.g., typically a cannulated,female-threaded axial rod plug 80, is next inserted, as follows: theproximal end of the plug 80 is fitted on the distal end of the roddriver that is configured (e.g., male hex head) to engage the plug 80,and which is then inserted and axially advanced into the proximal end ofthe proximal body 12 of the deployed mobility preservation apparatus 10.Next, the plug driver is removed. Once the plug driver has been removedfrom the access track, site closure is performed by the surgeon.

FIG. 14 illustrates an embodiment of a mobility preservation apparatus10 having an intermediate body 14 including a cable 170 as the flexibleelement 22 of intermediate body 14. The cable 170 of intermediate body14 is generally configured to limit the relative movement between of theproximal body 12 and the distal body 16. An expandable membrane 18, notshown, may be provided about cable 170. The expandable membrane 18 maybe sealingly secured to the distal end of the proximal body 12 and theproximal end of the distal body 16. The cable 170 is generally selectedto have the flex and compression properties desired for a particularapplication. For exemplary purposes, cable 170 is illustrated with acore 172 having a plurality of wires 174 spirally wound about the core172. The cable 170 may be manufactured from a material, such as forexample rubbers, silicones, plastics, metals, hard plastics, orcomposites thereof, among others. Typically, cable 170 is manufacturedfrom a biocompatible material.

FIG. 15 illustrates an embodiment of a mobility preservation apparatus10 having a having an intermediate body 14 including a plurality ofstacked washers 160 as a flexible element 22. The stacked washers 160are generally configured to limit the relative movement between of theproximal body 12 and the distal body 16. The stacked washers 160 aregenerally positioned about a cable 170. The cable 170 may be selected tohave the flex and compression properties which in cooperation with theparticular washer configuration result in the desired properties for themobility preservation apparatus 10. For exemplary purposes, cable 170 isillustrated with a core 172 having a plurality of wires 174 spirallywound about the core 172. An expandable membrane 18, not shown, may beprovided about cable 170. The expandable membrane 18 may be sealinglysecured to the distal end of the proximal body 12 and the proximal endof the distal body 16. The washers may be arranged along cable 170 toalternate between compressible washers 162 and non-compressible washers164, as is illustrated in FIG. 14 for exemplary purposes. Thecompressible washers 162 may be manufactured from a material having adesired degree of compressibility and elasticity, such as for examplerubbers, silicones, plastics, among other materials or combinations ofmaterials. The non-compressible washers 164 may be manufactured from amaterial having a desired degree of rigidity, such as for example metal,hard plastics, among other materials or combinations of materials.Typically, both the compressible washers 162 and the non-compressiblewashers 164 are manufactured from biocompatible materials.

FIG. 16 illustrates an embodiment of a mobility preservation apparatus10 having an intermediate body 14 including a flexible element 22 in aball and track configuration to provide support while allowing relativemovement of the proximal body 12 and the distal body 16. The ball andtrack configuration of flexible element 22 may further provide a degreeof translation in the anterior-posterior plane.

Generally, the ball and track configuration may include a distallyopening concavity 94 defined on a distal end of proximal body 12 and aproximal opening concavity 98 defined on a proximal end of distal body16 and a movable radially symmetrical pivot 100. Radially symmetricalpivot 100 is illustrated in the figures as a ball for exemplarypurposes. Radially symmetrical pivot 100 received within the each of thedistally opening concavity 94 and the proximal opening concavity 98 tomaintain the relative positions of the proximal body 12 and the distalbody 16. In one aspect, the intermediate body 14 including a ball andtrack configuration may include an expandable membrane 18 or otherbarrier to prevent material from entering the mobility preservationapparatus and disrupting the function of the ball and trackconfiguration.

As illustrated in FIG. 16, the ball and track configuration for aflexible element 22 includes a first insert 92 defining a distallyopening concavity 94 which forms a track to receive a radiallysymmetrical pivot 100 and a second insert 96 defining a proximal openingconcavity 98 to simultaneously receive the radially symmetrical pivot100. The radially symmetrical pivot 100, first insert 92 and secondinsert 96 are typically formed from a biocompatible material that isabrasion or wear resistant. The first insert 92 may be configured to bereceived within the proximal lumen 42 of the proximal body 12. Thesecond insert 98 may be configured to be received within the distallumen 46 or distal cavity 48 of distal body 16. In one aspect, the firstinsert 92 and second insert 96 are further configured to be receivedinto the proximal body 12 and distal body 16, respectively, in a mannerwhich allows motion in and out of the anchor but does not allowrotation. Moreover, the first insert 92 and second insert 96 mayinterface with the proximal body 12 and distal body 16 in a manner whereelastomeric bumpers, bushings, or shock absorbers reside at theinterfaces. In one embodiment, an insert anchor 102 may be providedwithin the proximal lumen 42 of the proximal body to secure the firstinsert 92, second insert 96 and ball 100 within the apparatus.

Situated in between the distally opening concavity 94 and the proximalopening concavity 98 is radially symmetrical pivot 100 that may beformed from the same materials as the proximal body 12 and distal body16 and/or first insert 92 and second insert 98. The radially symmetricalpivot 100 resides in the distally opening concavity 94 and the proximalopening concavity 98 and may be free to roll and translate within andalong one or more elongated concavities which is oriented such that itslong axis is substantially parallel to the anterior-posterior axis ofthe human body. The radially symmetrical pivot 100 may function toreceive at least a portion of the weight transferred through theproximal body 12 and the distal body 16. Each of the respectiveconcavities is typically situated in substantially parallel alignmentwith the corresponding adjacent vertebral endplate and form bearingsurface for forces transferred from the spine at least in part throughthe radially symmetrical pivot 100.

In one aspect, one of the distally opening concavity 94 and the proximalopening concavity 98 may be spherical and the other cavity may bespherical and elongated. The distally opening concavity 94 and theproximal opening concavity 98 may also be configured such that theirlengths are the same, and also they are not symmetrical about thecenterline, rather, they are offset in the anterior-posterior plane andalso oriented to be offset in directions that are opposite with respectto each other, e.g., with the L5 component being offset in the posteriordirection and the S1 component being offset in the anterior direction.In this manner, the opposing offsets of elongated concavities may allowthe ball 100 to roll anteriorly in flexion and posteriorly in extension.

Referring to FIG. 16 a, there is illustrated a first insert 92 from aball and track embodiment as described above. In the illustratedembodiment, the distally opening concavity 94 may be characterized by alongitudinal axis 104 and a transverse axis 106. The length of thedistal opening concavity 102 along the longitudinal axis may vary fromno less than about 50% of the diameter of the corresponding ball to asmuch as 3 or 4 times the diameter of the corresponding ball. Generally,the longitudinal length of the distal opening concavity 102 will bewithin the range of from about equal to the diameter of thecorresponding ball to about 3 times the diameter of the correspondingball. In embodiments having a ball with a diameter within the range offrom about 5 mm to about 15 mm, the length of the distal openingconcavity 102 along the longitudinal axis 104 will often be no greaterthan about the diameter of the ball plus 5 mm.

The distally opening concavity 94 comprises a generally cylindricalsection 108 having a substantially constant radius of curvature. Theradius of the cylindrical section 108 may be varied, depending upon thedesired lateral range of motion through the ball and track embodiment,as will be apparent to those of skill in the art in view of thedisclosure herein. In general, the radius of cylindrical section 108 maybe substantially the same as the radius of the corresponding ball, in anembodiment designed to restrict lateral translation across the mobilitypreservation device. The radius of the cylindrical section 108 may beincreased to at least about 105% or 110% or 120% or more of the radiusof the ball, in an embodiment in which lateral movement is desirable.

The distal opening concavity 102 is also defined by a first end 110 anda second end 112. In the illustrated embodiment, the first and secondends 110, 112 are provided with constant radius curvature which conformssubstantially to a portion of the surface of the ball. Alternatively,the radius of the first and second ends, as measured in a plane whichintersects the longitudinal axis 104 and bisects the first insert 92 maybe greater than the radius of the ball. In this construction, the firstand second ends, in cooperation with the weight of the patient, willprovide a dampening effect on the ends of the range of motion of thefirst insert 92 along longitudinal axis 104 with respect to thecorresponding distal insert.

The first insert 92 and corresponding distal insert and ball subassemblymay be assembled in place, such by advancing the components sequentiallyalong the access pathway. Alternatively, the opposing inserts and ballmay be provided as a subassembly, for insertion as a single unit. Thismay be accomplished, for example, by providing a tubular polymericmembrane around at least a portion of each of the proximal insert anddistal insert, and spanning the region of the ball. Encapsulating theball and track in a barrier membrane additionally serves to isolate theball and track from tissue ingrowth and body fluids.

In another aspect of the present invention, a plug 80 may be provided tobe received within proximal lumen 42 and/or intermediate lumen 44.Exemplary embodiments of plug 80 are illustrated in FIGS. 13, 17 and 18.As illustrated, plug 80 is configured to be inserted into the caudad endof proximal lumen 42. Plug 80 may be provided with a stiffening member82. FIGS. 17 and 18 illustrate plug 80 and stiffening member 82 as anintegral component for exemplary purposes. Plug 80 may be provided topreclude leakage or migration of the osteogenic, osteoconductive, orosteoinductive gel or paste, any fluid prosthetic nucleus material whichmay have been inserted into the disc space 860 following a nucleectomyprocedure or after implantation of a mobility preservation apparatus 10as well as any other fluid or discharge from the nucleus. Accordingly,the plug 80 is typically configured to be secured within the proximallumen 42 at a location caudad to the most caudad positioned fenestration74 although the plug 80 may be secured in the intermediate lumen 44, thedistal lumen 46 or the distal cavity 48 in certain configurations. Inone aspect, plug 80 provides a seal to substantially prevent fluidmigration through proximal lumen 42. External threads 84 may be providedto secure plug 80 in proximal lumen 42. As illustrated, external threads84 may be rotatably received by the internal threads 50 of lumen 40. Asillustrated for exemplary purposes, plug 80 is configured to be insertedusing a hex-head driver. Plug 80 may be fabricated from stainless steel,other suitable metals or alloys, or suitable polymeric material as willbe recognized by those skilled in the art.

The stiffening member 82 may also or alternatively be secured within alumen of mobility preservation apparatus 10. Stiffening member 82 may beprovided with external threads 84 to secure the stiffening member 82within proximal lumen 42 and/or intermediate lumen 44. When extendinginto the intermediate lumen 44, stiffening member 82 may stiffen theintermediate body 44 to convert the mobility preservation apparatus 10functionally into a fusion apparatus depending on how much the mobilitypreservation apparatus 10 is stiffened by the addition of the stiffeningmember 82. In one aspect, the intermediate body may be stiffened byhaving the stiffening member 82 extend to or beyond the flexible element22 of the intermediate body 14. As illustrated, external threads 84 maybe rotatably received by the internal threads 50 of proximal lumen 42.As illustrated for exemplary purposes, stiffening member 82 isconfigured to be inserted using a hex-head driver, which may beconfigured as a driver 400 illustrated in FIGS. 22 to 26. Stiffeningmember 82 may be configured to increase the stiffness of a mobilitypreservation apparatus 10. Further, a stiffening member 82 may functionto abut fenestrations 74 to preclude any fluid leakage. The stiffeningmember 82 may be configured in a variety of shapes and sizes and madefrom a wide range of materials to achieve the desired modification tothe flex properties of a mobility preservation apparatus. Stiffeningmember 82 may be fabricated from stainless steel, other suitable metalsor alloys, or suitable polymeric material as will be recognized by thoseskilled in the art. A series of distinct stiffening members 82 may beindicated to provide a progression of therapy in a single patient overtime or a single stiffening member 82 may be provided at the time ofimplantation of the mobility preservation apparatus 10. For example, aplug 80 having an integral stiffening member 82 inserted intointermediate lumen 44 may extend sufficiently through and distally intothe intermediate lumen 44 to reduce the flexibility of the intermediatebody 14 so that it effectively serves as an immobilization or fusiondevice. In certain aspects, the ability to later introduce a stiffeningmember 82 may allow for a progression in treatments for a patient. Forexample, a stiffening member 82 and/or plug 80 inserted through theproximal lumen 42 of the proximal body 12 into the intermediate lumen 44of intermediate body 14 through at least a portion of the flexibleelement 22. The stiffening member 82 and/or plug 80 may extend asufficient distance into the flexible element 22 such that the effectiveflexibility of the apparatus 10 is compromised. With the stiffeningmember 82 positioned within the flexible element 22, the apparatus 10may now serve as an immobilization or fusion device, particularly if,for example, osteogenic materials were first introduced through thefenestrations 74 into the intervertebral disc space 860. In anotheraspect, the apparatus 10 may be revisable, and also convertible. Thatis, the apparatus 10 in accordance with the present inventions may beexplanted, replaced, or converted in the progression-of-treatmentoptions from a prosthetic nucleus device to a total disc replacement, upto and including fusion. In addition to the stiffening member 82 and/orplug 80, the apparatus 10 may include multiple, modular components whichfacilitate such progression-of-treatment as are generally illustratedthroughout the figures. In accordance with these aspects, the apparatus10 may be further distinguished from prior spinal fusion devices whereaccompanying osteogenesis results in permanent immobilization of themotion segment.

As illustrated in FIGS. 19 and 20, a collar 518 may be deployed aboutthe expandable membrane 18 of the mobility preservation apparatus 10.Typically, the collar 518 is positioned radially about the expandedexpandable membrane in disc space 860. The collar 518 is generallyconfigured to modify the expansion characteristics of the expandablemembrane 18. In one aspect, the collar 518 may be configured toreinforce the annulus. The collar 518 may be integral with theexpandable membrane or may be a separate component. When integral, thecollar 518 may be formed as a thickened region of material around theexpandable membrane 18 and may be integrally formed. Alternatively, anintegral collar 518 may be mechanically and/or chemically attached tothe expandable membrane 18 such as for example with an adhesive. Whenseparate, the collar 518 may be positioned in situ prior to implantationalthough collar 518 may be configured on or within expandable membrane18. In one aspect, the collar 518 is silicone or polyurethane and isconfigured to conform to the annulus to prevent migration or leakage ofthe membrane through herniations. Accordingly, the collar 518 istypically stiffer than the expandable membrane 18. In a one embodiment,the collar is from between about 8 mm-12 mm high and about 0.5 to 1.0 mmthick. In operation, the collar 518 may be configured to abut theannulus from the inside and to share load with the annulus under higherstress conditions and also be strong enough to buttress any weak orherniated portions of annulus.

FIGS. 22 and 26 illustrate an exemplary embodiment of a driver 400 toposition an embodiment of a mobility preservation apparatus 10 within apatient. As illustrated, the driver 400 includes an elongated driveshaft 402 having a distal drive head 404. A proximal drive head 406 mayalso be provided at a proximal position along drive shaft 402. Distaldrive head 404 and proximal drive head 406 are illustrated with ahexagonal shape in axial cross section for exemplary purposes. Distaldrive head 404 and proximal drive head 406 may be generally configuredto be received by or to receive an aspect of a mobility preservationapparatus 10 to confer a rotational force for the purposes of implantingor positioning the mobility preservation apparatus. Accordingly, driveshaft 404 may be configured to confer a rotational force to drive head404 where the rotational force originates from a location outside of thepatient.

As discussed above and illustrated in FIGS. 24, 26, 27, 28, 30, 33, and34, the mobility preservation apparatus 10 may be introduced via theaxial pathway or bore 850 through at least one vertebral body of acaudal vertebra 800, across an intervertebral disc space 860, and into avertebral body of a cephalad vertebra 900. Here, the cephalad end of themobility preservation apparatus 10 may be inserted into place by adriver 400 engaged to the caudad end of the apparatus, and by manuallyturning it like a screw. In a multi-piece apparatus, the mobilitypreservation apparatus 10 may be rotated into place by first rotating onthe caudad end of the distal body 16 of the apparatus into the cephaladvertebra 900 prior to insertion of the intermediate body 14 and proximalbody 12. This insertion method as just described may be used for bothapparatus deployment and/or for distraction. After the distal body 16 isseated into the axial pathway or bore 850 through a vertebral body, themobility preservation apparatus 10 may be rotated in the cephaladdirection to cause the second threads 36 to screw into the wall of theaxial pathway or bore 850. The diameter of the axial bore 850 may bematched to the largest minor thread diameter of the mobilitypreservation apparatus 10. Generally, the mobility preservationapparatus 10 has been properly positioned when at least a portion of theintermediate body 14 and/or expandable membrane 18 is positioned withinthe disc space 860. However, proper positioning will vary depending uponthe particular embodiment of the mobility preservation apparatus 10 andthe condition of the patient.

In another aspect of the present invention, a mobility preservationapparatus 10 may be provided with a prosthetic nucleus either by formingthe prosthetic nucleus in situ or by inserting a pre-formed prostheticnucleus 200 as illustrated in FIGS. 30 to 32. The prosthetic nuclei maybe manufactured from a wide range of prosthetic nucleus materials 60such as for example silicone, polyurethane, hydrogel, among others. Ifusing a hydrogel, polyethylene glycol (PEG) is an example of a hydrogelwith adequate physical properties. In one aspect, the prosthetic nucleusmaterial 60 approximates the physiological properties (e.g.,biomechanical; hydrophicity) of natural nucleus. Accordingly, theprosthetic nucleus material 60 may have a Shore D range from between A10to D90. One exemplary prosthetic nucleus material 60 for the mobilitypreservation apparatus may include a cross-linked hyaluronic acid.Similarly, many natural and man-made hydrogels can be configured toachieve similar properties.

A prosthetic nucleus 200 may be formed in situ within the disc space 860after or prior to implantation of a mobility preservation apparatus 10.In one embodiment, the mobility preservation apparatus 10 includes aprosthetic nucleus formed from a prosthetic nucleus material 60 infusedinto the disc space 860. By flowing into the disc space 860, theprosthetic nucleus material 60 may fill the disc space. Further, theprosthetic nucleus material 60 may conform to and match the disc space860 created during the nucleotomy procedure. As illustrated in FIGS. 26to 29 and 34, the formation of a prosthetic nucleus 200 about a mobilitypreservation apparatus 10 in situ may include the introduction of aliquid or granular prosthetic nucleus material 60 through proximal lumen42 of a mobility preservation apparatus 10 and into the disc space 860through fenestrations 74 or through proximal lumen 42 of the mobilitypreservation apparatus 10. The prosthetic nucleus material 60 may beintroduced into the mobility preservation apparatus 10 with an elongatedintroducer 500 which is positioned to introduce the prosthetic nucleusmaterial 60 into the lumen 40 of the mobility preservation apparatus 10or directly into fenestrations 74, as illustrated in FIG. 30. After thedisc space 860 has been adequately filled, a plug 80 or a distractionrod may be secured within the lumen 40 to prevent the prosthetic nucleusmaterial 60 from leaking from the disc space 860. Alternatively, theprosthetic nucleus material 60 may polymerize within the disc space 860or the lumen 40 may be otherwise sealed to eliminate the need forplugging the proximal lumen 42 and/or fenestrations 74. In otheraspects, it may be preferable to seal the access bore 850 in the sacrumin order to prevent the escape of the prosthetic nucleus material 60. Avariety of configurations of plugs and/or seals known in the art can beused to seal the access bore 850, such as, for example, absorbable ornon-absorbable threaded plugs, conventional poly-methylmethacrylate(PMMA) bone cement, autologous or allograft bone dowels, andcombinations thereof. This approach is most applicable in situationswhere the annulus is fully intact, or has been otherwise sealed topreclude potential leaking or migration of material from within the discspace 860 through annular fissures, lesions, or herniations.

In another variation of the present invention, the previously notedspatial discrepancy between the mobility preservation apparatus 10 andthe remaining nucleus or annulus is obviated by prosthetic nucleusmaterials which are deployed directly into the intervertebral disc space860 to augment or replace the intervertebral disc, after which theaccess tract is sealed in order to retain the prosthetic nucleusmaterial by precluding migration or expulsion More specifically,prosthetic nucleus material 60, independent of a containment membrane orenvelope, is introduced directly into the prepared intervertebral discspace 860, when the annulus is known to be intact. In this embodiment,the prosthetic nucleus material serves as the “air” in the “tire” of thedisc, and is selected based on, among other attributes,biocompatibility; resistance to fractional mass loss over time and henceability to provide or maintain distraction over time; distribute loads(hydrostatic pressure in the disc), for example, to restore tensile hoopstress in the annulus. The prosthetic nucleus material 60 preferablycomprises biomedical grade hydrogels or blends thereof (e.g.,hydrogel/hydrogel, or hydrogel/elastomer). Cross-linked hyaluronic acid,such as is available from Fidia Corporation in Italy, is an example of asuitable material, however, many natural and man-made hydrogels orblends thereof may be configured to achieve similar properties withoutinflammatory response. The efficacy of this embodiment for its intendedfunction is further predicated on the requirement that the access siteto the disc space 860 is sealed to preclude prosthetic nucleus materialmigration or expulsion. Any one of numerous valve configurations, e.g.,self-sealing valve assemblies or flow-stop devices may suitably servethis function. Materials suitable for anchoring are similarly suitableas plugs, such as non-absorbable threaded plugs, including thosefabricated from medical grade polyether-ether-ketone (PEEK) such as thatcommercially available from Invibio Inc., in Lancashire, United Kingdom,or polyether-ketone-ketone PEKK available from Coors-Tech Corporation,in Colorado, or alternatively, conventional polymethylmethacrylate(PMMA); ultra high molecular weight polyethylene (UHMWPE), or othersuitable polymers in combination with autologous or allograft bonedowels may be used as plugs.

One advantage of this embodiment as just described would be theconcomitant use of prosthetic nucleus material 60 comprising hydrogelsas in situ delivery vehicles for a range of therapeutic compounds suchas, for example, progenitor cells analgesic and the like (e.g.,capitalizing on their biodegradability, through chemical hydrolysis oras enzymatically catalyzed). Moreover, certain bioactive hydrogels(e.g., such as those based on photo-crosslinked poly(ethylene oxide)[PEO], or block polypeptide or amino acid hydrogels), may be used in oneaspect of the present invention to engineer tissue, e.g., cartilage; orto serve as a dimensional matrix that promotes nerve regeneration, orreduces scar tissue formation.

Once positioned, a prosthetic nucleus may be formed in situ by expandingthe expandable membrane 18 with a prosthetic nucleus material 60 asgenerally illustrated in FIGS. 26, 27, 29, and 34. In one aspect, theprosthetic nucleus material 60 approximates the physiological properties(e.g., biomechanical; hydrophicity) of natural nucleus. For example, oneappropriate prosthetic nucleus material 60 for the mobility preservationapparatus may include a cross-linked hyaluronic acid. Similarly, manynatural and man-made hydrogels can be configured to achieve similarproperties.

As is generally illustrated in the FIGS. 26 to 29 and 34, a prostheticnucleus may be formed in situ within the disc space 860 afterimplantation of a mobility preservation apparatus 10. In one embodiment,the apparatus 10 includes a prosthetic nucleus formed from a prostheticnucleus material 60 infused into the disc space 860. By flowing into thedisc space 860, the prosthetic nucleus material 60 may fill the discspace. Further, the prosthetic nucleus material 60 may conform to andmatch the disc space 860 created during the nucleotomy procedure. Asillustrated in FIGS. 8 and 9, the formation of a prosthetic nucleus 200about a mobility preservation apparatus 10 in situ may include theintroduction of a liquid or granular prosthetic nucleus material 60through proximal lumen 42 of a mobility preservation apparatus 10 andinto the disc space 860 through fenestrations 74 of the mobilitypreservation apparatus 10. The prosthetic nucleus material 60 may beintroduced into the apparatus with an elongated introducer 500 which ispositioned to introduce the prosthetic nucleus material 60 into thelumen 40 of the mobility preservation apparatus 10 or directly intofenestrations 74. After the disc space 860 has been adequately filled, aplug 80 or distraction rod 60 may be secured within the lumen 40 toprevent the prosthetic nucleus material 60 from leaking from the discspace. Alternatively, prosthetic nucleus material 60 may polymerizewithin the disc space 860 or the lumen 40 may be otherwise sealed toeliminate the need for plugging the lumen 40 and or fenestrations 74. Inother aspects, it may be preferable to seal the access hole 850 in thesacrum in order to prevent the escape of the prosthetic nucleus material60. A variety of configurations of plugs and/or seals known in the artcan be used to seal the access hole 850, such as, for example,absorbable or non-absorbable threaded plugs, conventionalpoly-methylmethacrylate (PMMA) bone cement, autologous or allograft bonedowels, and combinations thereof. This approach is most applicable insituations where the annulus is fully intact, or has been otherwisesealed to preclude potential leaking or migration of material fromwithin the disc space 860 through annular fissures, lesions, orherniations.

If there is a tear or fissure in the annulus, it could be repaired usinga device that is passed through the dilator sheath following thecreation of a channel into the disc space 860 (using bone dilation,drilling, or a combination of the two). The device could tunnel throughthe disc material and deliver a glue (such as cyanoacrylate or a fibrinbased glue) substance to bolster the internal lamina of the discannulus. Such a delivery device could be made out of several materialsincluding nitinol tubing, or a flexible plastic material such aspolyethylene. The device would have to fit within the dilator sheath(Sheath ID=0.355″) and be at least 10″ in length to track from outsidethe body to the site of application. The ID of the delivery device needsto be large enough to allow insertion of the osteogenic promotermaterial which may very in viscosity (chunks or paste) depending on theneed. A flexible tube is inserted through the dilator sheath, oralternatively through a fenestrated plug or rod into the disc space 860to deliver such augmentation material.

In yet another aspect of the present invention, a preformed prostheticnucleus 200 can be deployed in the disc space 860 as illustrated inFIGS. 30 to 32. Once deployed, the preformed prosthetic nucleus mayoccupy at least a portion of the disc space 860 to provide additionalload bearing surface and capability, shock absorbing capacity, andadditional cushioning and resistance in motion. In this aspect of thepresent invention, mobility preservation apparatus 10 may include apreformed prosthetic nucleus 200. In general, the preformed prostheticnucleus 200 interfaces in contact with proximal and distal vertebralbodies after implantation about a mobility preservation apparatus 10.

With reference to FIGS. 30 to 32, in accordance with one aspect of thepresent invention, there is provided a preformed prosthetic nucleus 200for replacing or augmenting the spinal disc nucleus comprising asuperior surface 532 for contacting an adjacent, superior vertebral bodyend plate 533, an inferior surface 534, for contacting an adjacent,inferior vertebral body end plate 535, and a radial surface 536, forfacing in the direction of an annulus 408. In one embodiment, thesuperior and inferior surfaces 532, 534 preferably have a compliancysufficient to permit conformation to the respective, adjacent end plates533, 535, and the physical characteristics of the preformed prosthesis200 are selected such that a first portion or component of an axialcompression force exerted on the prosthesis 200 is transferred acrossthe prosthesis 200 to the superior and inferior end plates 533, 535,while a second portion of the axial compression force is deflectedlaterally in the direction of the annulus 408 (and/or via means of anintervening elastomeric collar) to allow load sharing by the annulus.

The outer wall of the preformed prosthetic nucleus 200 preferablyincludes a compliant membrane. With reference to FIGS. 11 a, 12 a and 12b, in one embodiment, compliancy of the radial surface 536 is less thanthe compliancy of at least one of the superior and inferior surfaces532, 534. In another embodiment, the wall supporting the radial surface536 is semi-compliant, e.g., is stiffer or has a greater thickness thanthe walls supporting the superior and inferior surfaces 532, 534.

In yet another embodiment, the preformed prosthetic nucleus 200 mayinclude a collar 518 carried by or within the radial surface 536. Anexemplary embodiment of an collar 518 is illustrated in FIGS. 19 and 20.As illustrated, the pre-formed prosthesis resumes its pre-formed shapeonce implanted inside the treatment site, such as, for example, the discspace.

In one embodiment, shown in FIG. 31, the prosthesis 200 is preformed,designed, or biased in a manner so that the prosthesis 200 naturallyassumes a partially open donut or ring structure or shape, where theleading and distal ends 540, 542 of the prosthesis 200 are located neareach other. Here, the leading end 540 and distal end 542 move and remainclose to each other without actually touching, thereby leaving a spacein between them, so that the prosthesis 200 assumes a C-shape with theends 540, 542 close to each other. In another embodiment, notillustrated, the ends 540, 542 come into contact with each other,thereby closing the gap between the ends 540, 542 so that prosthesis 200forms a closed donut or ring structure

With reference to FIG. 30, in one embodiment, one or more sections ofthe prosthesis 200 can be temporarily straightened or re-shaped for thepurpose of introducing the prosthesis 200 into the treatment site, suchas, for example, the disc space 860 resulting from a nucleotomy ordiscectomy procedure. With reference to FIG. 32, the prosthesis 200preferably re-forms or resumes its natural shape. In this particularembodiment, the natural shape is a donut or ring shape. It will beunderstood, the natural shape or bias of the preformed prosthesis 200can be any shape or configuration appropriate for a given application ortreatment. It will also be understood that the prosthesis 200 does notnecessarily assume the identical preformed shape or structure within thetreatment site. Here, variations in the shape of the prosthesis 200 inthe treatment site will depend on factors, such as, the area and shapeof any structures in the treatment site. However, in it will beunderstood that the preformed shape of the prosthesis 200 willpreferably be selected or determined with the shape, area, and/orgeometry of the treatment site in mind.

With reference to FIG. 30, in one approach, the method of implanting thepreformed prosthesis 200 into a treatment site includes the steps ofintroducing a modified distraction rod 550, in axial alignment with thelongitudinal axis of the vertebral motion segment using the anteriormethod into the treatment site. For this particular illustrativeembodiment, the treatment site includes a disc space 860 and portions ofthe superior and inferior vertebral bodies 800, 900 that abut the discspace 860. Here the modified distraction rod 550 includes a lumen 552extending along at least a portion of the length of the rod 550, anaperture 554 allowing communication between the lumen 552 and the discspace 860, and appropriately selected threads 556, 558 on the leadingand trailing sections of the rod 550 to achieve the desired degree ofdistraction of the vertebral bodies 800, 900. In one approach, thevertebral bodies are “over distracted” to facilitate the introduction ofthe preformed prosthesis 200 into the disc space 860. In anotherapproach, the vertebral bodies are distracted just enough to allow entryof the prosthesis 200 into the disc space.

The method of implanting the preformed prosthesis 200 into a treatmentsite may also include the steps of advancing the preformed prosthesis200 through the lumen 552 of the rod 550, and advancing the prosthesis200 into the disc space 860 through the aperture 554. In one approach,illustrated in FIG. 30, the prosthesis 200 is advanced or pushed throughthe lumen 552 of the rod 550 and into the disc space 860 by a push-rod.The push rod 560 is configured and dimensioned to fit within the lumen552 and has a prosthesis-engaging structure 562 on its leading end. Inone embodiment, shown in FIG. 12 a, the prosthesis-engaging structure562 includes a concave or cupped structure which engages with thetrailing end 542 of the prosthesis 530. Here, the prosthesis 200 isadvanced through the rod 550 and into the disc space 860 by pushing thepush-rod 560 in the cephalad direction. In this particular embodiment,the rod 550 includes a curved wall 555 that directs the prosthesis 200toward the aperture 554 and out into the disc space 860 as the 200prosthesis is advanced cephalad.

Once inside the disc space, the prosthesis 200 assumes its preformedshape—namely, a partially open donut shape in this particularillustrative embodiment (see FIGS. 11 a and 11 b). In one approach, therod 550 is removed so that the vertebral bodies 800, 900 settle towardeach other and onto the prosthesis 200, thereby causing the prosthesisto slightly bulge radially outward toward the annulus. It will beunderstood that the materials and dimensions of the preformed prosthesis200 are selected in a manner so that the prosthesis mimics theproperties of natural, healthy nucleus material. The material anddimensions of the prosthesis will depend on numerous factors, such as,for example, the size of the disc space 860 or treatment site, the sizeand anatomical characteristics of the patient, etc.

In accordance with another aspect of the present invention, there isprovided a mobility preservation apparatus 10 comprising an elasticsleeve structure illustrated in FIGS. 33 and 34. The elastic sleeveprosthesis 400 may have a proximal body 12, a proximal body 16, and anintermediate body 14 which are substantially continuous in structure.The prosthesis 400 comprises a mesh structure. The prosthesis 400 can mebe made of any number of suitable materials known in the art, such as,for example, an elastomer. It will be understood that the material(s) ofused to form the mesh structure of the mobility preservation apparatus10 are preferably semi-compliant. As used herein, semi-compliant refersto biomechanical properties, e.g., modulus, that permit conformabilitywith the pertinent vertebral structures but that do not render thedevice too compressible such that it would be able to extrude from thedisc space 860 (e.g., through herniations in the annulus, or backthrough the deployment pathway) nor so stiff that loads are notdistributed to/shared by physiological structures (potentially leadingto, for example, transition syndrome). In another embodiment, theelastic sleeve 400 of the mobility preservation apparatus 10 comprises asolid or non-mesh structure. As with the mesh structure, the material(s)of the mobility preservation apparatus 10 are semi-compliant, that isbalanced to be firm yet flexible and resilient.

With reference to FIGS. 33 and 34, in one exemplary approach, theelastic sleeve 400 is inserted into the treatment site comprising a discspace 860 and sections of the two vertebral bodies 800, 900 abutting thedisc space 860. In one preferred approach, the elastic sleeve 400 isintroduced into treatment site after the vertebral bodies 800, 900 havebeen distracted. With reference to FIG. 34, after the elastic sleeve 400is inserted into the disc space 860 and any distraction tools ormechanisms (not shown) have been removed, the vertebral bodies 800, 900settle toward each other and onto the elastic sleeve 400 in a mannerthat causes the intermediate body 14 of the elastic sleeve 400 to bulgeradially outward toward the annulus 408 so that a superior surface 578,an inferior surface 580, and a radial surface 582 form near theintermediate body 14 of the elastic sleeve 400. Here, the superiorsurface 578 contacts or engages with a superior vertebral body end plate533, the inferior surface 580 engages with an inferior vertebral bodyend plate 535, and the radial surface 582 faces in the direction of theannulus 408. In one embodiment, the radial surface 582 engages withnucleus material between the elastic sleeve 400 and the annulus 408. Inanother embodiment, the radial surface 582 engages directly with theannulus 408.

With continued reference to FIG. 34, the prosthesis 400 provides axialcompressive strength at the treatment site. In one embodiment, a firstportion or component of an axial compression force exerted on theprosthesis 400 is transferred across the prosthesis 400 to the superiorand inferior end plates 533, 535, while a second portion of the axialcompression force is deflected laterally in the direction of the annulus408 to allow load sharing by the annulus 408. In one preferredembodiment, the elastic sleeve 400 abuts the annulus 408, therebyfacilitating the transfer a portion of any axial compression force(s) aslateral force(s) for absorption by the annulus 408. While the presentinvention has been illustrated and described with particularity in termsof preferred embodiments, it should be understood that no limitation ofthe scope of the invention is intended thereby.

While the present invention has been illustrated and described withparticularity in terms of preferred embodiments, it should be understoodthat no limitation of the scope of the invention is intended thereby.Features of any of the foregoing methods, and exemplary devices shownand briefly described below, may be substituted or added into theothers, as will be apparent to those of skill in the art. The scope ofthe invention is in no way intended to be limited by the brevity orexemplary nature of the material below, and may be further understoodfrom the accompanying Figures. It should also be understood thatvariations of the particular embodiments described herein incorporatingthe principles of the present invention will occur to those of ordinaryskill in the art and yet be within the scope of the materials describedand shown herein.

Features of any of the foregoing methods and devices may be substitutedor added into the others, as will be apparent to those of skill in theart. It should also be understood that variations of the particularembodiments described herein incorporating the principles of the presentinvention will occur to those of ordinary skill in the art and yet bewithin the scope of this disclosure.

1. A method for supporting the spine, comprising the steps of: implanting a mobility preservation apparatus between a cephalad vertebral body and a caudad vertebral body, the mobility preservation apparatus comprising an intermediate body, wherein the intermediate body permits the relative flexing of the cephalad vertebrae and the caudad vertebrae that are adjacent to the mobility preservation apparatus; inserting a stiffening member into a spinal implant in a preformed axial bore extending along a longitudinal axis of the spine in the caudad vertebral body; and positioning the stiffening member within at least a portion of an intermediate lumen defined by the intermediate body to increase the stiffness of the intermediate body.
 2. A method, as in claim 1, further comprising the step of the stiffening member converting the mobility preservation apparatus to fusion apparatus.
 3. A method for supporting the spine, comprising the steps of: providing a mobility preservation apparatus having an intermediate body, the intermediate body having a proximal end and a distal end, the intermediate body including at least one flexible element positioned between the proximal end and the distal end and an intermediate lumen; forming an axial bore extending along a longitudinal axis of the spine through a caudad vertebral body, an intervertebral disc and a cephalad vertebral body; removing at least a portion of a nucleus pulposus of the intervertebral disc through the axial bore to form a disc space; axially inserting the mobility preservation apparatus through the axial bore; securing at least a portion the mobility preservation apparatus in the cephalad vertebral body with a first retention structure; securing at least a portion of the mobility preservation apparatus in the caudad vertebral body with a second retention structure; and positioning a stiffening member within at least a portion of the intermediate lumen to control the stiffness of the at least one flexible element.
 4. A method, as in claim 3, further comprising the step of the stiffening member converting the mobility preservation apparatus to fusion apparatus.
 5. A method, as in claim 3, further comprising the step of securing the stiffening member within the intermediate lumen.
 6. A method, as in claim 3, further comprising the step of rotating the stiffening member within the intermediate lumen to secure the stiffening member within the intermediate lumen.
 7. A method, as in claim 3, further comprising the step of providing the mobility preservation apparatus comprising the intermediate body configured as substantially radially symmetrical about a longitudinal axis.
 8. A method, as in claim 3, further comprising the step of providing the mobility preservation apparatus comprising the intermediate body defining at least one fenestration.
 9. A method, as in claim 7, further comprising the step of providing a mobility preservation apparatus comprising a proximal body, the proximal body configured as substantially radially symmetrical about a longitudinal axis, the proximal body secured to the proximal end of the intermediate body, and the proximal body including a first retention structure.
 10. A method, as in claim 9, further comprising the step of providing a mobility preservation apparatus comprising the intermediate body defining at least one fenestration.
 11. A method, as in claim 10, further comprising the step of providing a mobility preservation apparatus comprising the proximal body defining a proximal lumen in fluid communication with the at least one fenestration of the intermediate body.
 12. A method, as in claim 9, further comprising the step of providing a mobility preservation apparatus comprising a distal body, the distal body configured as substantially radially symmetrical about a longitudinal axis, the distal body secured to the distal end of the intermediate body, and the distal body including second retention structure.
 13. A method for supporting the spine, comprising the steps of: providing a mobility preservation apparatus having an intermediate body being substantially radially symmetrical about a longitudinal axis and having a proximal end and a distal end, the intermediate body including at least one flexible element positioned between the proximal end and the distal end, and the intermediate body defining at least one fenestration, a proximal body being substantially radially symmetrical about the longitudinal axis, the proximal body secured to the proximal end of the intermediate body, the proximal body including a first retention structure, and the proximal body defining a proximal lumen in fluid communication with the at least one fenestration of the intermediate body, and a distal body being substantially radially symmetrical about the longitudinal axis, the distal body secured to the distal end of the intermediate body, and the distal body including a second retention structure; forming an axial bore extending along a longitudinal axis of the spine through a caudad vertebral body, an intervertebral disc and a cephalad vertebral body; removing at least a portion of a nucleus pulposus of the intervertebral disc through the axial bore to form a disc space; axially inserting the mobility preservation apparatus through the axial bore; securing at least a portion of the distal body of the mobility preservation apparatus in the cephalad vertebral body with the first retention structure; securing at least a portion of the proximal body of the mobility preservation apparatus in the caudad vertebral body with the second retention structure; and positioning a stiffening member within at least a portion of the intermediate lumen to control the stiffness of the at least one flexible element.
 14. A method, as in claim 13, further comprising the step of the stiffening member converting the mobility preservation apparatus to fusion apparatus.
 15. A method, as in claim 13, wherein positioning a stiffening member within at least a portion of the intermediate lumen includes increasing the stiffness of the flexible element.
 16. A method, as in claim 13, wherein axially inserting the mobility preservation apparatus through the axial bore includes inserting prosthetic nucleus through the axial bore.
 17. A method, as in claim 3, wherein positioning a stiffening member within at least a portion of the intermediate lumen includes increasing the stiffness of the flexible element.
 18. A method, as in claim 3, wherein axially inserting the mobility preservation apparatus through the axial bore includes inserting prosthetic nucleus through the axial bore.
 19. A method, as in claim 1, wherein implanting a mobility preservation apparatus includes implanting a mobility preservation apparatus through the axial bore.
 20. A method, as in claim 1, wherein implanting a mobility preservation apparatus includes inserting a prosthetic nucleus through the axial bore. 